Tuning the index of a waveguide structure

ABSTRACT

The index of refraction of waveguide structures can be varied by altering carrier concentration. The waveguides preferably comprise semiconductors like silicon that are substantially optically transmissive at certain wavelengths. Variation of the carrier density in these semiconductors may be effectuated by inducing an electric field within the semiconductor for example by apply a voltage to electrodes associated with the semiconductor. Variable control of the index of refraction may be used to implement a variety of functionalites including, but not limited to, tunable waveguide gratings and resonant cavities, switchable couplers, modulators, and optical switches.

PRIORITY APPLICATION

[0001] This application is a divisional of U.S. patent application Ser.No. 10/242,318, entitled “Tuning the Index of a Waveguide Structure”,filed on Sep. 10, 2002 which claims priority under 35 U.S.C. § 119(e)from U.S. Provisional Patent Application Serial No. 60/318,486, entitled“Tunable Resonant Cavity Based on the Field Effect In Semiconductors,”filed Sep. 10, 2001, U.S. Provisional Patent Application Serial No.60/327,137, “High Speed Optical Modulator Based on CMOS CompatibleTunable Resonant Cavity” filed Oct. 4, 2001, and U.S. ProvisionalApplication Serial No. 60/328,474, entitled “Technique for Tuning theIndex of an Optical Structure and Use of this Effect for Tuning theCoupling,” filed Oct. 11, 2001, as well as U.S. Provisional PatentApplication Serial No. 60/318,445 entitled “SOI Waveguide withPolysilicon Gate” and filed Sep. 10, 2001, each of which is herebyincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is directed to semiconductor devices, andmore particularly to controlling the propagation of photons throughsemiconductor structures.

[0004] 2. Description of the Related Art

[0005] Light offers many advantages when used as a medium forpropagating information, the foremost of which are increased speed andbandwidth. In comparison with electrical signals, signals transmittedoptically can be switched and modulated faster and can include an evengreater number of separate channels multiplexed together. Accordingly,lightwave transmission along optical fibers is widespread in thetelecommunications industry. In an exemplary fiber optic communicationsystem, a continuous wave (CW) beam of light may be emitted from a laserdiode and modulated using an electro-optical modulator that is driven byan electrical signal. This electrical signal may correspond to voice ordata which is to be transmitted over a distance between, e.g., twocomponents in a computer, two computers in a network, or two phonesacross the country or the world. The light travels in an optical fiberto a location where it is detected by an optical sensor, which outputsvoltage that varies in accordance with the modulation of the opticalbeam. In this manner, information can be rapidly transported from onelocation to another. To increase data throughput numerous opticalsignals at different wavelengths can be multiplexed and transmittedtogether along a single optical path. This optical path can be switchedselectively and varied to direct the optical signals to the appropriatedestination.

[0006] In constructing optical systems, such as the one described above,a variety of functionalities are desirable. One useful element is amodulator for varying a specific property of light such as amplitude orphase. Another valuable component is a tunable filter for selectivelyisolating certain optical frequencies. Additional useful elements arecouplers and switches for controllably transferring light from one pathto another. What is needed are advantageous designs and techniques formodulating and filtering light as well as for coupling and switchingoptical signals from one path to another.

SUMMARY OF THE INVENTION

[0007] In one aspect of the invention, an apparatus comprises awaveguide, a tunable resonant cavity, and first and second electrodes.The tunable resonant cavity comprises a closed path for propagatingelectromagnetic waves, the close path comprising a semiconductor havinga distribution of free carriers. The closed path is juxtaposed with thewaveguide to permit the coupling of electromagnetic waves between thewaveguide and the closed path. The first and second electrodes arepositioned to apply an electric field through an insulator into thesemiconductor of the tunable resonant cavity. The distribution of freecarriers in the semiconductor is responsive to the electric field tovary phase delay introduced by the closed path.

[0008] In another aspect of the invention, an optical apparatus alsocomprises a waveguide, a tunable resonant cavity, and first and secondelectrodes. The tunable resonant cavity comprises a semiconductor havinga distribution of free carriers and a substantially circular opticalpath. The circular optical path is juxtaposed with the waveguide topermit the coupling of light between the waveguide and the circularoptical path. The first and second electrodes are positioned to apply anelectric field through an insulator into the circular optical path. Thedistribution of free carriers in the circular optical path is responsiveto the electric field to vary the optical path length of the circularoptical path.

[0009] Still another aspect of the invention comprises a method oftuning a resonant cavity. In this method an optical resonator comprisingsemiconductor is provided and an electric field is applied through aninsulator to at least a portion of the semiconductor to alter freecarrier distribution in said semiconductor. The resonant frequency ofthe optical resonator is thereby changed from a first frequency to asecond frequency.

[0010] Yet another aspect of the invention comprises an opticalapparatus comprising a first waveguide, a second waveguide, asemiconductor and first and second electrodes for applying an electricfield through an insulator to the semiconductor. The semiconductorprovides an optical path between the first and second waveguides tocouple light between the waveguides. The adjustment of the electricfield changes the free carrier density in the optical path such thatabsorption of light in the optical path increases, thereby decreasingthe coupling of light between the first and second waveguides.

[0011] Another aspect of the invention comprises an optical switchingapparatus comprising first and second waveguides and a carriercontrolled optical switch having at least first and second states. Thecarrier controlled optical switch comprises a coupling waveguide whichprovides an optical path between said first and second waveguides andfirst and second electrodes. The coupling waveguide provides an opticalpath between the first and second waveguides. The coupling waveguidecomprises a semiconductor having a refractive index dependent on adistribution of free carriers within the semiconductor. The first andsecond electrodes are for applying an electric field through aninsulator into the semiconductor. The distribution of free carriers isresponsive to application of the electric field to change the state ofthe carrier controlled optical switch from the first state to the secondstate.

[0012] Another aspect of the invention comprises a method of selectivelycoupling light between first and second waveguides. The method comprisesproviding a semiconductor positioned to couple light between thewaveguides along an optical path and changing free carrier density ofthe semiconductor in the optical path to alter coupling between thewaveguides.

[0013] Still another aspect of the invention comprises an opticalwaveguide coupler having a tunable coupling coefficient. The couplercomprises a first waveguide and a second waveguide juxtaposed forcoupling and a first electrode. The first waveguide is comprised ofsemiconductor having a distribution of free carriers. The firstelectrode is electrically connected to a first variable voltage sourcefor applying an electric field to the semiconductor of the firstwaveguide. The distribution of free carriers is responsive toapplication of the electric field to change the coupling coefficientbetween the waveguides from a first value to a second value.

[0014] Still another aspect of the invention comprises a method oftuning the coupling coefficient of an optical waveguide coupler. In thismethod, a first waveguide comprising semiconductor containing adistribution of free carriers is provided. A second waveguide juxtaposedto the first waveguide is also provided. An electric field is applied tothe semiconductor of the first waveguide to alter the free carrierdistribution in the semiconductor, thereby changing the couplingcoefficient of the optical waveguide coupler from a first value to asecond value.

[0015] Yet another aspect of the invention comprises a waveguide gratingcomprising a waveguide for propagating light in a longitudinaldirection. The waveguide comprises a plurality of elongate membersoriented transverse to the longitudinal direction. The members aredisposed relative to the waveguide to form a grating for coupling lightout of the waveguide. The waveguide has a carrier density at each of themembers. These members include respective electrodes for applying anelectric field to the waveguide, the electric field varying this carrierdensity in the waveguide such that the coupling is altered.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Preferred embodiments of the present invention are describedbelow in connection with the accompanying drawings.

[0017]FIG. 1 is a schematic illustration of an embodiment of an opticalswitching apparatus including a carrier controlled optical switchcomprising a resonant cavity disposed between two waveguides.

[0018]FIG. 2 is a cross-sectional view of a preferred embodiment of aresonant optical cavity formed with a disk-shaped semiconductor.

[0019]FIGS. 3 and 4 are top and perspective views, respectively, of theresonant optical cavity of FIG. 2.

[0020]FIG. 5 is a cross-sectional view of a preferred embodiment of aresonant optical cavity formed with an annular-shaped semiconductor.

[0021]FIGS. 6 and 7 are a top and perspective views, respectively, ofthe resonant optical cavity of FIG. 5.

[0022]FIGS. 8 and 9 are top views showing different electrodeconfigurations associated with preferred embodiments of a resonantoptical cavity.

[0023]FIGS. 10A and 10B are perspective and cross-sectional views of aresonant optical cavity schematically illustrating confinement of lightin the optical cavity.

[0024]FIGS. 10C and 10D are cross-sectional views of the resonantoptical cavity depicted in FIGS. 10A and 10B schematically illustratingoptical confinement by introducing an annular shaped strip around theperimeter of the optical cavity and doping the center of the opticalcavity.

[0025]FIG. 11 is a cross-sectional view of another resonant opticalcavity formed with a disk-shaped semiconductor configured to provide analternative carrier distribution.

[0026]FIG. 12 is a plot on axis of frequency (in arbitrary units) andoptical power (in arbitrary units) depicting the quality factorassociated with different states of a resonant optical cavity.

[0027]FIG. 13 shows plots illustrating the output variation over time infirst and second waveguides responsive to a modulation voltage appliedto an optical switching apparatus such as depicted in FIG. 1.

[0028]FIG. 14A is a perspective view of a directional coupler comprisinga pair of spatially separated waveguides brought within close proximityalong a coupling region. FIG. 14B shows a cross-sectional view throughthe line 14B-14B through the coupling region of the directional couplerof FIG. 14A.

[0029]FIG. 15A-15C are plots of light intensity as a function oflocation along the line 15-15 in FIG. 14B illustrating the extent ofcoupling for three different optical states.

[0030]FIGS. 16-19 are cross-sectional views of waveguides pairscomprising a directional coupler such as shown in FIG. 14A eachconfigured differently to selectively alter the optical states forvaried levels of optical coupling.

[0031]FIG. 20A shows a schematic top view of a waveguide adjacent to adisk-shaped optical resonator. FIG. 20B shows a cross-sectional view ofthe waveguide and optical resonator along a line 20B-20B in FIG. 20A.

[0032]FIG. 21 is a perspective view of a waveguide grating comprising aplurality of electroded rulings on a channel waveguide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0033] These and other embodiments of the present invention will alsobecome readily apparent to those skilled in the art from the followingdetailed description of the preferred embodiments having reference tothe attached figures, the invention not being limited to any particularembodiment(s) disclosed. Accordingly, the scope of the present inventionis intended to be defined only by reference to the appended claims.

[0034] In general, optical waveguides comprise a core region comprisingmaterial that is at least partially transparent. This core region issurrounded by a cladding region that confines light within the coreregion. Some optical energy, often referred to as the evanescent energyor the evanescent field, however, may exist outside the core region andwithin the cladding region.

[0035] In certain waveguides, the core region comprises a first mediumhaving a first refractive index, and the cladding region or claddingcomprises a second medium having a second refractive index, therefractive index of the core region being greater than the refractiveindex of the cladding region. A core/cladding interface is located atthe boundary between the core region and the cladding region. In suchembodiments, when light in the core region is incident upon thiscore/cladding interface at an angle greater than the critical angle, thelight is reflected back into the core region. This effect is referred toas total internal reflection. In this manner, optical signals can beconfined within the core region due to total internal reflection at thecore/cladding interface.

[0036] Waveguides can be fabricated in a wide variety of geometries andconfigurations. A channel waveguide, and more specifically, a buriedchannel or embedded strip waveguide, is a specific type of waveguidethat fits the description above. A channel waveguide generally comprisesa core comprising a first medium having a relatively high refractiveindex surrounded by a relatively lower refractive index cladding region.A buried channel or embedded strip waveguide generally comprises a coreembedded in a substrate that forms at least part of the surroundingcladding region.

[0037] A buried channel waveguide is an example of an integrated opticalwaveguide, which are generally associated with a substrate. Theintegrated optical waveguide may for example be situated on thesubstrate, in a substrate, or partially on and partially in thesubstrate. The integrated optical waveguide may be part of the substrateitself but preferably comprises one or more layers of materialpositioned on a surface of the substrate. Examples of integrated opticalwaveguides include the channel waveguides discussed above, as well asslab waveguides, rib or ridge waveguides, and strip loaded waveguides.

[0038] In accordance with conventional usage in the art, opticalcomponents that are integrated onto a substrate with integrated opticalwaveguides, are collectively referred to herein as integrated optics.Such optical component may for example, process, manipulate, filter orotherwise alter or control optical signals propagating within thewaveguides. As discussed above, these components themselves may bewaveguides that guide light.

[0039] One of the simplest integrated optical waveguide configurationsis the conventional slab waveguide. The slab waveguide comprises a thin,planar slab surrounded by cladding regions. The cladding regions maytake the form of first and second (for example, upper and lower)cladding layers on either side of the slab. The two cladding layers neednot comprise the same material. In this simplified example, the slab maybe planar with substantially parallel planar boundaries at theinterfaces with the first and second cladding layers. Generally, theslab has a higher refractive index than either of the cladding layers.Light can therefore be confined in one dimension (e.g., vertically)within the slab. In this configuration of the slab waveguide, opticalenergy is not confined laterally to any portion of the slab, but extendsthroughout the slab due to total internal reflection at the planarboundaries between the slab and the surrounding upper and lower claddinglayers.

[0040] A ridge or rib waveguide is formed by creating thicknessvariations in the slab. These thickness variations may be formed bydepositing material on selected regions of the slab or by removingmaterial from selected regions of the slab. The slab with the ridges orribs formed thereon may be surrounded on opposite sides by the first andsecond (e.g., upper and lower cladding layers) comprising relatively lowrefractive index material. The thicker portions, i.e., the ridges orribs, which comprise more slab material, will have a higher effectiveindex than thinner region of the slab which comprise relatively lesseramounts of the slab material.

[0041] Accordingly, the region within the slab that is beneath thethicker portions and in proximity thereto has a higher effectiverefractive index than other portions of the slab. Thus, unlike the slabwaveguide wherein optical energy propagates throughout the planar slab,the ridge or rib waveguide substantially confines optical energy to theregion of the planar slab layer within and under the ridge and inproximity thereto. In a ridge or rib waveguide, therefore, an opticalsignal can be propagated along a path in the slab defined by the regionunder which the ridge or rib is located on the slab. Thus, ridgewaveguides defining any number and variations of optical pathways can becreated by forming one or more ridges or ribs in the slab having theshape and orientation of the desired optical pathways.

[0042] Similarly, a strip loaded waveguide is formed by positioning astrip on the slab of a slab waveguide. The slab and the strip locatedthereon may be surrounded on opposite sides by the first and second(e.g., upper and lower) cladding layers having lower refractive indexthan the slab. Preferably, the strip has a refractive index that isgreater than that of either cladding layer, however, the index of thestrip is preferably approximately equal to that of the slab. Thepresence of the strip positioned on the slab induces an increase ineffective index of the slab in the region beneath the strip and inproximity thereto.

[0043] As with the ridge or rib waveguide, the region within the slabthat is beneath the strip and in proximity thereto has a highereffective refractive index than other portions of the slab. Thus, thestrip loaded waveguide substantially can confine optical energy to theregion of the planar slab layer under the high-index strip, some of theoptical energy also being within the strip itself. Accordingly, in astrip loaded waveguide an optical signal can be propagated along a pathin the slab defined by the region over which the high-index strip isplaced on the slab. Waveguides corresponding any number and variationsof optical pathways, can be created by depositing one or more stripsonto the slab having the shape and orientation of the desired opticalpathways.

[0044] Another form of waveguide discussed in U.S. patent applicationSer. No. 10/241,284 entitled “Strip Loaded Waveguide with Low-IndexTransition Layer” filed Sep. 9, 2002, which is hereby incorporatedherein by reference in its entirety, comprises a slab having a firstrefractive index n₁ and a strip having a second refractive index n₂. Inaddition, the strip loaded waveguide structure has a transition layerhaving a third refractive index n₃. The transition layer is positionedbetween the slab and the strip, such that the slab and the strip do notdirectly contact each other. The refractive index of the transitionlayer n₃ is less than the refractive index of the slab n₁ and therefractive index of the strip n₂.

[0045] The light within the slab is confined to portions beneath thestrip because of the presence of the strip, despite the fact that thestrip is separated from the slab. The intervening transition layer doesnot prevent the strip from determining the shape and location of theoptical mode(s) supported in the slab. The presence of the strippositioned proximally to the slab portion induces an increase ineffective index of the slab portion in the region directly under thestrip and in proximity thereto. This increase in effective index definesa relatively high effective index guiding region wherein light in one ormore supported optical modes is guided along the strip loaded waveguide.The strip loaded waveguide guides supported modes in the guiding regiondespite the presence of the transition layer between the slab and strip.In particular, the transition layer does not prevent the strip fromaltering the effective index within the slab and more particularly, fromraising the effective index within the slab. Preferably, the transitionlayer has a thickness sufficiently small such that the strip canincrease the effective index of the slab in regions immediately beneathand in the proximity thereto. The transition layer is sufficiently thinand the strip and the slab are sufficiently close, although physicallyseparated by the intervening transition layer, that the strip can affectthe propagation of light within the slab. The transition layer alsopreferably has an index of refraction that is low in comparison withthat of the strip and the slab.

[0046] In certain embodiments of the invention, semiconductor materialsused in conventional processes for fabrication of semiconductormicroelectronics are employed to create waveguide structures. Thesematerials include, but are not limited to, crystalline silicon,polysilicon and silicon dioxide (SiO₂). In particular, in variouspreferred embodiments of the strip load waveguide having an insulatingtransition layer, the slab comprises single crystal silicon, thetransition layer comprises silicon dioxide, and the strip comprisespolysilicon, although in other embodiments, the strip may comprisecrystal silicon. The crystal silicon slab and the polysilicon strip arepreferably doped so as to be electrically conducting although inportions of the slab or strip that are not to be conductive, the slaband the strip are preferably undoped to minimize absorption losses.

[0047] As is well known, single crystal silicon is used to fabricatesemiconductor microelectronics and integrated circuits (ICs), such asmicroprocessors, memory chips and other digital as well as analog ICs,and thus single crystal silicon is well characterized and its propertiesare largely well understood. The term single crystal silicon is usedherein consistently with its conventional meaning. Single crystalsilicon corresponds to crystalline silicon. Single crystal silicon,although crystalline, may include defects such that it is not truly aperfect crystal, however, silicon having the properties conventionallyassociated with single crystal silicon will be referred to herein assingle crystal silicon despite the presence of such defects. The singlecrystal silicon may be doped either p or n as is conventional.

[0048] Single crystal silicon should be distinguished from polysiliconor “poly”. Polysilicon is also used to fabricate semiconductormicroelectronics and integrated circuits. The term polysilicon or “poly”is used herein consistently with its conventional meaning. Polysiliconcorresponds to polycrystalline silicon, silicon having a plurality ofseparate crystalline domains. Polysilicon can readily be deposited forexample by CVD or sputtering techniques, but formation of polysliconlayers and structures is not to be limited to these methods alone.Polysilicon can also be doped p or n and can thereby be madesubstantially conductive. In general, however, bulk polysilicon exhibitsmore absorption losses in the near infrared portion of the spectrum thana similar bulk single crystal silicon, provided that the doping,temperature, and other parameters are similar.

[0049] Optical switches, modulators, and couplers, among other devices,can be implemented using various waveguide structures including but notlimited to the types discussed above, e.g., channel, slab, rib or ridge,strip-loaded, and strip loaded with transition layer. In addition, thesestructures can be formed using semiconductor materials, such as forexample, silicon.

[0050] A. Optical Switching Apparatus

[0051]FIG. 1 is a schematic diagram of an optical switching apparatus.The switching apparatus includes a carrier controlled optical switch 104that may be used to couple light between a first waveguide 100 and asecond waveguide 102.

[0052] The effective refractive index of the first and second waveguides100, 102 is larger than the effective refractive index of claddingregions 108 surrounding the waveguides so as to allow the waveguides100, 102 to propagate light in a guided fashion, as discussed above.

[0053] The carrier controlled optical switch 104 includes an opticalpath between the first waveguide 100 and the second waveguide 102. Invarious preferred embodiments, the optical path comprises a resonantcavity 106, preferably comprised of an optically transparentsemiconductor. More particularly, this resonant cavity 106 preferablycomprises a waveguide structure comprising semiconductor material. Theoptical path further comprises a first gap region, A, between the firstwaveguide 100 and the resonant cavity 106, and a second gap region, B,between the second waveguide 102 and the resonant cavity 106. The sizesof the gap regions, A, B, permits control of the coupling of lightbetween waveguides 100, 102 and the resonant cavity 106, and allows fora weak coupling of light, which is desirable under certain conditions.

[0054] Preferably, the resonant cavity 106 is configured to accumulateor deplete free carriers such as electrons and/or holes. The refractiveindex of the material comprising the resonant cavity 106 issignificantly larger than the refractive index of the confining region108 enabling light to be guided within the resonant cavity. Furthermore,the refractive index of at least a particular region within thesemiconductor 106 is variable, depending upon the density of freecarriers in that region.

[0055] The resonant cavity 106 further includes an electrode 110 forapplying an electric field through an insulator 112 into thesemiconductor 106. (The insulator preferably comprises silicon dioxide.)The electrode 110 is preferably metal or polysilicon, and is connectedto a variable voltage source 114 that may be used to control themagnitude of an electric field applied to the semiconductor 106.

[0056] These waveguides 100, 102 as well as the resonant cavity 106depicted schematically in FIG. 1, may comprise channel waveguides, ribor ridge waveguides, or strip loaded waveguides although the waveguidedesign should not be limited to these specific types. In one preferredembodiment, however, the waveguides 100, 102 comprise strip loadedwaveguides having a low-index transition layer between the strip and theslab described above as well as disclosed in in U.S. patent applicationSer. No. 10/241,284 entitled “Strip Loaded Waveguide with Low-IndexTransition Layer” filed Sep. 9, 2002.

[0057] These particular strip loaded waveguides comprises comprises aslab and a strip, wherein the strip is separated from the slab. A layerof material having an index of refraction lower than that of the stripand the slab is disposed between and separates the strip and the slab.Nevertheless, a guiding region is provided for propagating an opticalmode and this guiding region extends both within the strip and the slab.In certain embodiments, for example, the slab and strip comprisesemiconductor and the transition region comprises dielectric.

[0058] Application of a voltage between the semiconductor strip and theslab causes carriers to accumulate within the guiding region of thestrip loaded waveguide. For example, depending on the polarity of theapplied voltage and the doping, electrons or holes may accumulated or bedepleted within the semiconductor slab in a regions adjacent to the thintransition layer comprising dielectric material. The structure acts likea capacitor, charging with application of a voltage. The voltage createsan electric field across the thin transition layer with carriersaccumulating (or depleting) adjacent to this transition layer.

[0059] These strip loaded waveguides are preferably located on asupporting structure or substrate. The supporting structure serves tosupport the strip loaded waveguide and preferably comprises a materialsuch as silicon or sapphire. Additionally, the supporting structure mayalso include a cladding layer or layers, which aid in confining opticalenergy within the slab portion. Accordingly, this cladding preferablyhas a refractive index that is low in comparison to the refractive indexof the slab.

[0060] In one preferred embodiment, the supporting structure comprises asilicon substrate having a cladding layer of silicon dioxide formedthereon. The silicon dioxide layer on the silicon substrate with anindex of approximately 1.5 serves as a lower cladding layer for theslab. This silicon substrate may be doped.

[0061] Accordingly, the slab is disposed either on the substrate or on alayer (preferably the cladding) formed over the substrate. This claddinglayer itself may be formed directly on the substrate or may be on one ormore layers formed on the substrate. As discussed above, the slabpreferably comprises single crystal silicon and has an index ofrefraction n₁ on average of about 3.5 and a thickness t₁ preferablybetween about $\frac{\lambda}{6n}$

[0062] and $\frac{\lambda}{4n},$

[0063] and more preferably about $\frac{\lambda}{4n}.$

[0064] This thickness, t₁, determines in part the optical mode or modessupported by the strip loaded waveguide and depends partially on thegeometry of the structure. In alternative embodiments, the slab maycomprise materials other than single crystal silicon and may be doped orundoped and thus may have different refractive indices. The slab,however, preferably comprises crystal silicon. Localized doping, such asused to create the source, drain, and channel regions in a transistor,may cause the index of refraction in localized regions of the slab tovary slightly.

[0065] In general, the strip is disposed above and in a spaced-apartconfiguration with respect to the slab. The strip may comprise dopedpolycrystalline silicon having an index of refraction n₂ ofapproximately 3.5. In alternative embodiments, the strip may comprisedoped single crystal silicon having an index of refraction n₂ on averageabout 3.5. As discussed above, however, the strip may also be undopedand may comprise materials other than polysilicon or crystal siliconalthough these materials are preferred. An example of one suchalternative material that may be used to form the strip is siliconnitride, which has an index of refraction of approximately 1.9.

[0066] The dimensions of the strip may vary and depend in part on theoverall composition and geometry of the waveguide. As with the slab, thesize of the strip determines in part the number of modes to be supportedby the waveguide and the wavelength of these modes.

[0067] The transition layer is positioned between the slab and thestrip. Preferably, the refractive index of the transition layer is lessthan the refractive index of the polysilicon strip and the crystallinesilicon slab. In one preferred embodiment, the transition layercomprises silicon dioxide having an index of refraction n₃ ofapproximately 1.5.

[0068] The strip loaded waveguide is preferably covered at leastpartially by a coating although more than one coating or layers may beformed on the waveguide in various embodiments. This coating may provideelectrical insulation between separate conductive pathways. The coatingmay also serve as a cladding layer, providing confinement of opticalenergy within the slab and the strip. Accordingly, the coating orcoatings preferably has a composite index of refraction lower than thatof the slab and the strip. The coating may have an index of refractionequal to that of the low-index transition layer and may comprise thesame material as the low-index transition layer. Alternatively, thecoating may have a different index of refraction than the transitionlayer and may comprise different material. The coating preferablycomprises silicon dioxide. Other materials and, more specifically, otherdielectrics may also be employed.

[0069] Confinement of light within the slab is provided because the slabhas a higher refractive index than the layers above and below. In onepreferred embodiment, for example, light is confined within the siliconslab because the silicon slab has a higher refractive index than thesilicon dioxide coating covering it. In addition, the silicon slab has ahigher index than the silicon dioxide cladding layer immediately belowit. Lateral confinement within the slab is provided by the loadingcaused by the strip.

[0070] In this manner, light can be propagated through specific guidingregions within the slab. The guiding region corresponds to a boundarywhere a specific portion of the optical energy within the mode,preferably the fundamental mode, is substantially contained and thuscharacterizes the shape and spatial distribution of optical energy inthis mode. Accordingly, the guiding region corresponds to the shape andlocation of the optical mode or modes in this strip loaded waveguide. Inthe guiding region, the electric field and the optical intensity areoscillatory, whereas beyond the guiding region, the evanescent fieldexponentially decays.

[0071] As discussed above, these strip loaded waveguides may be employedto form resonant optical cavities, however, the resonant opticalcavities disclosed herein are only exemplary and different designs andmaterial systems, both those well known or yet to be devised, may beutilized in the alternative to create resonant optical cavities,modulators, couplers, switches or other related components.

[0072] B. Resonant Optical Cavity

[0073] A preferred embodiment of the resonant cavity 106 is illustratedin FIGS. 2 through 4. FIG. 2 is vertical cross-section through the line2-2 shown in FIG. 3. FIGS. 3 and 4 are top and perspective views of theresonant cavity 106, respectively. FIGS. 2, 5, 11, and 20, depictdesigns that include conformal metalization. Alternatively,planarization techniques can be used as is conventional in contemporarysemiconductor device fabrication.

[0074] The resonant cavity 106 comprises a disk-shaped slab 204 on topof a cladding layer 202 formed on a substrate 200. The disk-shaped slab204 preferably has higher index of refraction than the cladding layer202. In one preferred embodiment, the disk-shaped slab comprises crystalsilicon (e.g., active crystal silicon) and the cladding layer 202comprises a silicon dioxide layer (e.g., a buried-oxide layer) on asilicon substrate.

[0075] An insulating layer 206 covers the disk-shaped slab 204. Theinsulating layer 206 preferably has a refractive index lower than therefractive index of the disk-shaped slab 204, so as to act as an uppercladding layer confining light within the disk-shaped slab 204. Theinsulating layer 206 preferably comprises silicon dioxide, which has arefractive index substantially lower than the refractive index of singlecrystal silicon. The insulating layer 206 also prevents unwanted flow ofelectrical current between conducting elements of the device. Theinsulating layer 206 may comprise a plurality of layers, preferably lowindex dielectrics films overlaying on each other. Those of skill in theart would recognize that other insulating materials such as polymerslike polyamide may be used, provided they have appropriate opticalproperties.

[0076] An annular strip 208 comprising relatively high refractive indexmaterial is disposed over but space apart from the slap 204. Thisannular strip 208 follows a path around the outer portion of thedisk-shaped slab 204. The annular strip 208 preferably comprisesmaterial having an index of refraction that is high compared to that ofthe insulating layer 206 covering the disk-shaped slab 204. Thismaterial comprising the strip 208 is also preferably substantiallytransparent and non-absorbing to the wavelength light for which theresonant cavity 106 is designed. Preferably, the strip material issubstantially conductive and may comprise doped semiconductor. In onepreferred embodiment, the annular strip 208 comprises doped polysilicon,which has a refractive index comparable to that of single crystalsilicon. Alternatively, the strip 208 may comprise single crystalsilicon. Those of skill in the art would recognize that other materialsmay be used for the strip 208. The materials preferably have asubstantially high refractive index in comparison with the insulatingmaterial covering the disk-shaped slab 204.

[0077] The strip 208 is separated from the disk-shaped slab 204 by atransition layer 216 of the insulating material to prevent the flow ofcurrent between the strip 208 and the disk-shaped slab 204 to therebyfacilitate carrier accumulation and depletion. This insulating materialpreferably has a lower index of refraction than the disk-shaped slab 204as well as the annular shaped strip 208. This transition layer 216preferably has sufficiently thickness such that the carriers do nottraverse this barrier either through defects (e.g., “pin hole” defects)or by tunneling. Conversely, the thickness of this dielectric layer 216is preferably not so large as to require an excessively highly voltageto be applied to the device to generate or deplete the desired amount ofcarriers. In one preferred embodiment, this transition layer 206comprises silicon dioxide.

[0078] As shown in FIG. 2, the resonant cavity 106 further includes afirst (strip) electrode 210 electrically connected to the annular strip208. As shown in FIGS. 3 and 4, the first electrode 210 includes asubstantially annular portion that is electrically connected to avoltage source 220. This voltage source may be an AC or DC voltagesupply depending on the particular application. This embodiment furtherincludes a second (slab) electrode 212 electrically coupled to a centraltop surface of the disk-shaped slab 204. This central portion of thedisk-shaped slab 204 preferably includes a doped region 214 electricallycontacting the slab electrode 212 so as to create an ohmic contactbetween the disk-shaped slab 204 and the slab electrode 212. Asillustrated in FIG. 4, the second electrode 212 is also electricallycoupled to the voltage source 220 allowing for the application of apotential difference between the strip and slab electrodes 210, 212although other configurations for establishing an electric field acrossthe transition layer 216 are possible. The strip and slab electrodes210, 212 are separated by the insulating layer 206 to prevent unwantedelectrical contact therebetween; see FIG. 2. The insulating layer 206 isnot shown in FIGS. 3 and 4 in order to allow illustration of theinterior features of the resonant cavity structure 106. The strip andslab electrodes 210, 212 preferably are comprised of metal, although oneof skill in the art would recognize that other materials, such as dopedpolysilicon, may be used. Salicide may also be included to formed afavorable electrical contact to semiconductor regions. In particular,ohmic contacts can be formed between a metal electrode and an underlyingsalicide region in the semiconductor. In this manner, for example, theslab electrode 212 can be electrically connected to the semiconductorslab 204.

[0079] As discussed above, the strip 208 will confine the light toregions within the slab 204 beneath the strip and in proximity theretoas a result of the effect of the strip on the effective index of theslab. A portion of the optical power will also be contained within thestrip 208 as well as the transition layer 216. The thickness of the slab204 and the strip 208 as well as the width of the strip will in partdetermine the optical mode or modes that are supported, their spatialextent, and the associated wavelengths. Preferably, these dimensions areselected so as to support a single mode such as the “whispering gallery”mode which travels within the disk shaped slab 204 around its perimeter.

[0080] The first electrode 210 is also preferably about as wide as thewidth of the optical mode confined below the strip 208. The surface areaof the first electrode 210 for a resonator with a free spectral rangeequivalent to approximately a 50 nanometers (nm) optical communicationsband and supporting the resonator mode for a given wavelength is roughlya few square microns. This small size for the first electrode 210 allowsfor very high speed modulation due to the small associated capacitance.

[0081] The structure 106 shown in FIGS. 2-4 may be manufactured usingconventional integrated circuit fabrication processes. For instance, thesupporting structure may comprise a commercially available silicon waferwith silicon dioxide formed thereon. Conventional “Silicon-on Oxide”(SOI) processes can be employed to form the silicon slab on the siliconwafer or on a sapphire substrate. Fabrication techniques for forming acrystal silicon layer adjacent an insulator include, but are not limitedto, bonding the crystal silicon on oxide, SIMOX (i.e., use of ionimplantation to form oxide in a region of single crystal silicon), orgrowing silicon on sapphire. Oxide formation on the silicon slab can beachieved with conventional techniques used in field effect transistor(FET) technology for growing gate oxides on a silicon active layers.Still other processes utilized in fabricating FETs can also be applied.In the same fashion that a polysilicon gate is formed on the gate oxidein field effect transistors, likewise, a polysilicon strip can be formedover the oxide transition region in the waveguide structure. Thispolysilicon strip can be patterned using well-known techniques such asphotolithography and etching. Damascene processes are also consideredpossible. Accordingly, conventional processes such as those employed inthe fabrication of Complementary Metal Oxide Semiconductor (CMOS)transistors can be used to create the resonant cavity 106. In otherembodiments, crystalline silicon strips can be formed on the transitionoxide region using conventional techniques such as SOI processing.

[0082] Another strategy for fabricating the strip loaded waveguide is toobtain a commercially available SOI wafer which comprises a firstsilicon substrate having a first silicon dioxide layer thereon with asecond layer of silicon on the first silicon dioxide layer. Theaggregate structure therefore corresponds to Si/SiO₂/Si. The firstsilicon dioxide layer is also referred to as the buried oxide or BOX. Asecond silicon dioxide layer can be formed on the SOI wafer andpolysilicon or silicon strips can be formed on this structure to createthe resonant cavity 106 with the second silicon layer corresponding tothe disk-shaped slab 205 and the second silicon dioxide layer formedthereon corresponding to the insulating transition layer. The thicknessof this second silicon dioxide transition layer can be controlled asneeded. The polysilicon or silicon strips can be patterned for exampleusing photolithography and etching. Damascene processes are alsoenvisioned as possible.

[0083] In the case where the substrate does not comprise silicon with alayer of silicon dioxide on the surface, a slab comprising crystalsilicon can still be fabricated. For example, crystalline silicon can begrown on sapphire. The sapphire may serve as the lower cladding for theslab. Silicon nitride formed for example on silicon can also be acladding for a silicon slab. The formation of the transition layer andthe strip on the silicon slab can be performed in a manner as describedabove.

[0084] Other conventional processes for forming layers and patterningmay also be used and are not limited to those specifically recitedherein. Employing conventional processes well known in the art isadvantageous because the performance of these processes is wellestablished. SOI and CMOS fabrication processes, for example, are welldeveloped and well tested, and are capable of reliably producing highquality products. The high precision and small feature size possiblewith these processes should theoretically apply to fabrication ofstrip-loaded waveguides as the material systems are similar.Accordingly, extremely small sized waveguide structures and componentsshould be realizable, thereby enabling a large number of such waveguidesand other components to be integrated on a single die. Althoughconventional processes can be employed to form the waveguides describedherein, and moreover, one of the distinct advantages is thatconventional semiconductor fabrication processes can readily be used,the fabrication processes should not be limited to only those currentlyknown in art. Other processes yet to be discovered or developed are alsoconsidered as possibly being useful in the formation of thesestructures.

[0085] One additional advantage of these designs is that in variousembodiments electronics, such as transistors, can be fabricated on thesame substrate as the waveguide structures. Integration of waveguidesand electronics on the same substrate is particularly advantageousbecause many systems require the functionality offered by bothelectronic, optical, electro-optical, and optoelectronic components. Forexample, resonant cavities, modulators, switches, and other waveguidestructures, can be optically connected together in a network ofwaveguides and electrically connected to control and data processingcircuitry all on the same die. The integration of these differentcomponents on a single die is particularly advantageous in achievingcompact designs.

[0086] Another preferred embodiment for the resonant cavity 106 is shownin FIGS. 5 through 7. FIGS. 5-7 show the same views provided for theresonant cavity 106 illustrated in FIGS. 2-4, namely, cross-sectional,top, and perspective views. The cross-sectional view of FIG. 5 is acrossthe line 5-5 shown in FIG. 6.

[0087] The resonant cavity 106 illustrated in FIGS. 5-7 has a slab 304like the disk-shaped slab 204 in the resonant cavity shown in FIG. 2-4,however, this slab has a hole therein. Accordingly, the slab 304 isannular and may be characterized as a channel-like waveguide instead ofa slab-like waveguide or as a hybrid of the two waveguide types.Nevertheless, this portion 304 of the structure 106 will be referredherein as a slab region with the understanding that it has a holetherein which may act to confine light within an annular region. Theslab region 304 sits atop a cladding layer 302 formed on a substrate300. The slab region 304 preferably has a higher index of refractionthan the cladding layer 302. In various preferred embodiments, the slabregion 302 comprises single crystal silicon (e.g., active silicon) andthe cladding layer 302 comprises silicon dioxide (e.g., buried-oxidelayer). Other semiconductors may be used provided they are sufficientlytransparent in the wavelength range of interest.

[0088] As shown in FIG. 5, this embodiment further includes aninsulating layer 306 at least partially covering the slab 304. Thisinsulating layer 306 preferably has a refractive index lower than therefractive index of the slab 304, so as to serve as an upper claddinglayer confining light within the slab. The insulating layer 306preferably comprises silicon dioxide. As discussed above, the insulatinglayer 306 prevents unwanted flow of electrical isolated conductingpathway that form part of the structure 106. Those skilled in the artwill recognize that other insulating materials such as polymers may beused in forming the insulating layer 306. This insulating layer 306 maycomprise a plurality of sub-layers.

[0089] This structure preferably further includes an annular strip 308comprising a relatively high refractive index material substantiallyaligned with the annular shaped slab 304. This material preferably has arelatively high refractive index in comparison to that of the insulatinglayer 306 covering the slab 304. The material comprising the annularstrip 308 is also preferably substantially transparent and non-absorbingto the wavelength light for which the resonant cavity 106 is designed.Preferably, the strip material is partially conductive and may comprisedoped semiconductor. In certain preferred embodiments, the annular strip308 comprises doped polysilicon, which has a refractive index comparableto that of single crystal silicon. Alternatively, the annular strip 308may comprise single crystal silicon. Those of skill in the art wouldrecognize that other materials may be used for the strip 308. Thematerials preferably have a substantially high refractive index incomparison with the insulating material covering the slab 304. Althoughthe strip 308 is shown as having an outer perimeter substantiallyaligned with that of the annular shaped slab 304, in other embodiments,the two need not be aligned. Furthermore, the slab 304 may extend wellbeyond the strip 308 especially in cases where the slab is not circularor annular but is a sheet or layer of material or unpatterned bulksubstantially wider than the total spatial extent of the annular strip.

[0090] The strip 308 is separated from the slab 304 by a transitionlayer 316 of the insulating material to prevent the flow of currentbetween the strip 308 and the 304 to thereby facilitate carrieraccumulation and depletion. This insulating material preferably has alower index of refraction than the slab 304 as well as the annularshaped strip 308. This transition layer 316 preferably has sufficientlythickness such that the carriers do not traverse this barrier eitherthrough defects (e.g., “pinhole” defects) or by tunneling. Conversely,the thickness of this dielectric layer 316 is preferably not so large asto require a large voltage to be applied to the device to generate ordeplete the desired amount of carriers.

[0091] As shown in FIG. 5, the resonant cavity 106 further includes afirst (strip) electrode 310 electrically coupled to the annular strip308. As shown in FIGS. 6 and 7, the strip electrode 310 includes aportion that is annular in shape that is electrically connected to avoltage source 320. This embodiment further includes a second (slab)electrode 312 electrically coupled to the top surface of the slab 304.As discussed above, the slab 304 has a hole therein and is likewiseannular with an inner and an outer diameter and respective concentricboundaries defined by inner and outer edges. Similarly, the annularstrip 308 has an inner and an outer diameters and respective concentricboundaries defined by inner and outer edges. Preferably, the innerdiameter of the annular strip 308 exceeds the inner diameter of theannular slab 304. The slab 304 extends radially inward beyond the inneredge of the annular strip 308 so as to expose an annular-shaped topsurface of the slab 304 for connection with the slab electrode 312. Thisinner portion of the slab 304 preferably includes a doped region 314 incontact with the annular-shaped portion of the slab electrode 312 so asto create an ohmic contact between the resonant cavity 304 and the slabelectrode 312.

[0092] As shown in FIG. 7, the slab electrode 312 is also electricallycoupled to the voltage source 320, allowing for the application of apotential difference between the strip and slab electrodes 310, 312.This voltage source may be AC or DC. Alternate sources of power forcreating an electric field between the strip 308 and the slab 304 arealso possible. As shown in FIG. 5, the first and second electrodes 310,312 are separated by the insulating layer 306 to prevent unwantedelectrical contact between them. The insulating material 306 is notshown in FIGS. 6 and 7 in order to more clearly illustrate of theinterior features of the structure. The strip and slab electrodes 310,312 preferably comprise a metal, although one of skill in the art wouldrecognize that other materials may be used such as for example,polysilicon or silicide.

[0093] As discussed above with respect to FIGS. 2-4, the structure shownin FIGS. 5-7 may be manufactured using conventional fabricationprocesses including but not limited to SOI and CMOS technology.Deposition and patterning techniques may include for example,sputtering, chemical vapor deposition, etching, and damascene processes,which are well known in the art of semiconductor device fabrication aswell as fabrication methods yet to be developed.

[0094] In various other embodiments, the shape of the resonant cavitymay be configured differently. For example, the annular slab 304 shownin FIGS. 5-7 may be narrower such that for example the inner diameters(as well as outer diameters) of the annular slab and annular strip 310are substantially the same. In this exemplary case, the slab 304 doesnot extends radially beyond the edges of the annular strip 308 so as toexpose an annular-shaped top surface of the slab 304. Electricalconnection is made elsewhere to the slab 304. The narrower width of theslab 304 may act to confine the optical mode laterally. As discussedabove, this annular guiding structures 304 may provide lateralconfinement and for this reason is like a channel type waveguide incontrast with a slab waveguide, confining light both in vertical andhorizontal directions, even without the presence of the annular strip308. This configuration is referred herein as “ring-shaped.”

[0095] In certain embodiments, the annular strip 208, 308 and slab 204,304 may also be shaped differently so as to provide a closed opticalpath other than circular or annular. Other geometries for guiding lightare also possible. In addition, the resonant optical cavity path may notbe completely closed but may include interruptions, for example, wherelight can escape or be coupled into or out of the resonant cavity. Inother embodiments, the optical path may not be closed at all, and maymore closely resemble a Fabry-Perot resonant cavity with reflectivesurface on opposite ends, the light propagating back and forth ratherthan round and round a closed optical path. As discussed above, theresonant cavities can be formed using waveguides such as for example,strip loaded, channel, ridge or rib, and slab. These resonant cavity mayalso be formed from photonic crystal band gap waveguides or may compriseother types of guiding structures known in the art or yet to bedeveloped.

[0096] The electrode configuration may also be configured differently.The optical resonator shown in FIGS. 2-4 includes a first electrode 210that forms an uninterrupted continuous ring above the perimeter regionof disk-shaped slab 204 and annular strip 208. FIG. 8 shows a top viewof another embodiment for an optical resonator 106 in which the firstelectrode 410 only forms a partial ring, extending around less than theentire circumference of the resonant cavity 404. Metal is absorbing andmetallization and/or salicide in the proximity of the guiding region mayinduce attenuation of light therein. Accordingly, it is desirable toreduce the interaction between the metal electrodes and the opticalfield to avoid or reduce absorption of light by the metal. Theconfiguration of the strip electrode 410 in FIG. 8 advantageouslydecreases the amount of light absorbed by this electrode. In thestructure shown in FIG. 8, the resonant cavity 106 comprises a diskshaped slab 404 and an annular strip 408 similar to the slab 204 andstrip 208 depicted in FIGS. 2-4. The strip 408 forms a circular closedpath substantially following the perimeter region of the slab 404. Inthis case, the outer boundary of the slab 404, hidden in FIG. 8, issubstantially aligned with the outer boundary of the strip 408 althoughalignment is not necessary. For example, the slab 404 may extend beyondthe perimeter of the strip 408 especially in other embodiments where theslab comprises a sheet of material having large spatial extent incomparison with that of the strip 408. The first strip electrode 410 iselectrically coupled to the strip 408 and the second slab electrode 412is electrically coupled to the slab 404. Preferably, the strip electrode410 is spaced from the slab 404 by the strip 408 in a similar fashion asthe strip 408 shown in FIGS. 2-4. The slab electrode 412 is separatedfrom the strip 408 by insulating material, e.g., oxide. The insulatingmaterial separating the strip 408 from the slab electrode 412,especially at the location where the electrode passes over the strip, ispreferably sufficiently thick to reduce or avoid interaction between theslab electrode and the optical mode within the strip. Separation is alsodesirable to avoid shorting the slab electrode 410 on the conductingstrip 408.

[0097] In FIG. 8, the slab electrode 412 corresponds to the slabelectrode 212, 312 shown in FIGS. 2-4 and FIGS. 5-7, respectively,permitting the application of a controllable voltage between the firstand second electrodes 410, 412. Various other features discussed abovein connection with the previous embodiments, such as the presence of thedoped region 214 in the slab 204, may be present in this structure fromof FIG. 8 as well.

[0098] Other embodiments of the resonant cavity may include two or moreelectrode segments positioned above the perimeter region of the cavity.For example, FIG. 9 shows a top view of an embodiment of an opticalresonator in which the first electrode 510 includes three spaced apartsections substantially extending along perimeter portions of theresonant cavity. In FIG. 9, the resonant cavity 106 comprises a diskshaped slab 504 and an annular strip 508 similar to the disk shaped slab204 and annular strip 208 of FIGS. 2-4. The strip 508 forms circularclosed path substantially following the perimeter region of the slab504. (In this case, the outer boundary of the slab 504, hidden in FIG.9, is substantially aligned with the outer boundary of the strip 508.)The first strip electrode 510 is electrically coupled to the strip 508and the second slab electrode 512 is electrically coupled to the slab504. Preferably, the strip electrode 510 is spaced from the slab 504 bythe strip 508 as are the corresponding strips 208, 308 shown in FIGS.2-4 and 5-7. The slab electrode 512 is separated from the strip 508 byinsulating material, e.g., oxide. The insulating material separating thestrip 508 from the slab electrode 512, particularly at the locationwhere the electrode passes over the strip, is preferably sufficientlythick to reduce or avoid interaction between the slab electrode and theoptical mode within the strip. Separation is also desirable to avoidshorting the slab electrode 410 on the conducting strip 508. The stripelectrode 510 includes connecting portions, omitted from FIG. 9 forclarity, that electrically connect the three illustrated portions. Theseconnecting portions are spaced above the portions of strip electrode 510shown in FIG. 9, and are separated from strip 508 by insulatingmaterial. The segmented strip electrode 510 permit electric fields to beapplied to designated regions of the annular strip 508. Absorptionresulting from interaction of a segmented metal/salicide strip electrode510 and the optical field within the strip 508 are also reduced bydecreasing the area of interaction between the strip electrode and thestrip. Segmented electrodes are also compatible with conventional CMOSfabrication processes and designs which employ a plurality of vias downto, e.g., salicide layers.

[0099] The strip electrode 510 and the slab electrode 512 shown in FIG.9, are connected to a supply (not shown) to permit the application of avoltage between the strip and the slab. Various other features discussedabove in connection with the previous embodiments, such as the presenceof the doped region 214 in the resonant slab 204, may also be present inthis structure as well.

[0100] C. Operation of the Optical Resonant Cavity

[0101] The operation of the optical cavity 106 shown in FIGS. 2-4 willnow be described. Under certain conditions, when the resonant cavity 106of FIG. 2 is disposed sufficiently close to a waveguide propagatinglight, such as the first waveguide 100 of FIG. 1, light from thewaveguide may couple into the disk-shaped slab 204 of the resonantcavity. Because the disk-shaped slab 204 has a substantially higherrefractive index than the upper and lower cladding 206, 202 above andbelow, light can confined therein. Lateral confinement within the slab204 is provided by the annular strip 208 which defines a substantiallycircular path around the perimeter region within the slab. Light coupledinto the resonant cavity 106 propagates around this closed optical pathpartially within the slab and partially within the strip. The opticalmode will likely be distributed within the strip, the region of the slabsubstantially below the strip and in proximity thereto, as well aswithin the transition region 216 therebetween.

[0102] Light traveling on a closed path within the resonant cavity 204can interfere constructively or destructively with itself depending uponthe length of the closed path, the wavelength of the light, and theeffective index of refraction along that path. More particularly, thecontrolling relationship is between the wavelength and the optical pathlength (OPD) of the optical path in the resonant cavity, i.e., productof the physical length of the path and the effective index of refractionalong that path. Light traveling on paths for which the total opticalpath length is an even number of half-wavelengths will experienceconstructive interference; light traveling on paths for which the totaloptical path length is an odd number of half-wavelengths will experiencedestructive interference. Because of this phenomenon, the resonantcavity 204 contains one or more standing waves at certain frequenciesassociated with different modes.

[0103] It is generally known that the m^(th) resonant frequency, ν_(m),of a generic resonant cavity is given by$v_{m} = \frac{m\quad c}{n_{eff}l}$

[0104] where m is the mode number (an integer), c is the speed of lightin a vacuum, n_(eff) is the effective index of refraction of the mode inthe resonator, and l is the path length of a full round trip inside thecavity. This equation applies to optical resonant cavities in general.

[0105] For a resonant cavity having a circular optical path, thecircumference of the cavity determines the resonant wavelength. For theresonator depicted in FIGS. 2-4, the optical power for the optical modeis concentrated in a narrow band around the perimeter portion of theslab 204 beneath the strip 208 and in proximity thereto. Significantoptical power in this optical mode is also present within the annularstrip 208, and within the portion of the insulating material 216 locatedbetween the strip 208 and the slab 204.

[0106] Modulation of the resonant frequency of the optical resonantcavity 106 may be achieved by changing the effective index of refractionof the material comprising the perimeter portion of the cavity 204.Changing the index changes the effective optical path length, n_(eff)l,thus changing the resonant frequency as dictated by the above equation.

[0107] The effective index of refraction of a mode is proportional tothe real refractive index, n₀, such that:

n_(eff)=n_(r)n₀

[0108] where n_(r) depends upon the geometry of the waveguide. Thechange in the resonant frequency, Δν, due to a change in the refractiveindex, Δn, is given by:${\Delta \quad v} = {{- \frac{v_{m}}{n_{0}}}\Delta \quad {n.}}$

[0109] This equation applies a broadly to optical resonant cavities ingeneral.

[0110] As discussed above, the refractive index of a semiconductor, suchas silicon, is dependent upon the existence of free carriers within thesemiconductor, such that increasing the number of free carriers in aregion generally lowers the refractive index of that region. Conversely,decreasing the number of free carriers in a region raises the refractiveindex of that region. Thus, by manipulating the number of free carriersin a region of a semiconductor like single crystal or polycrysallinesilicon, the refractive index of that region may be controlled. Changingthe refractive index changes the effective optical path length of thecavity, n_(eff l), and, by extension, the resonant frequency, ν_(m).Accordingly, the resonant cavity can be tuned.

[0111] The density of free carriers in a region of the disk-shaped slab204 beneath the strip 208, and within the strip as well, may be changedvia the field effect by applying a potential difference between thestrip and slab electrodes 210, 212. As used herein the term “fieldeffect” corresponds to the effect exhibited in field effect transistors(FETs). Application of an electric field to a semiconductor junctioncauses a depletion of carriers near the junction. With continuedapplication of the field, inversion may result wherein opposite typecarriers are attracted to the junction and the depletion region. In thismanner, the free carrier distribution in the semiconductor can becontrolled and varied by applying an electric field to thesemiconductor. This junction may be formed between the semiconductor andan adjacent insulator across which the electric field is applied.

[0112] In this case, applying a voltage between the strip and slabelectrodes 210, 212 through the insulating transition region 216 createsan electric field that may cause electrons to be depleted at the topsurface of the slab 204 beneath the annular strip 208, and moreparticularly, beneath the insulating transition layer 216. Thisdepletion of electrons occurs in the case where the semiconductor isdoped n-type and the a polarity is appropriate to force these electronsaway from the junction. Applying additional voltage between the 208strip and the slab electrodes 210, 212 may cause inversion wherein holesare attracted and accumulate at the portion of the slab 204 beneath thestrip and the transition layer 216. The existence of the insulatingtransition layer 216 prevents the holes from flowing into to the strip208.

[0113] The existence of the insulating transition layer 216 allows forthe manipulation of the optical properties of the resonant cavity 204using the field effect, a variation of which is utilized in field effecttransistors (FET) technology, such as metal-oxide on semiconductor fieldeffect transistors (MOSFET).

[0114] The field effect enables modulation and/or control of the freecarriers and free carrier density underneath the strip 208, preciselywhere the optical mode is confined, thereby providing strong interactionbetween the carriers and the light. Increasing the magnitude of theapplied voltage increases the depletion or accumulation of eitherelectrons or holes, depending on the polarity and doping, and otherconditions. Accordingly, the effective index of refraction can bechanged. This ability to variably control the refractive index permitstuning of the resonant frequency of the cavity.

[0115] Because metals strongly absorb light, it is advantageous to keepthe strip and slab electrodes 210, 212 at a distance from the opticalpath of the resonant cavity formed by strip 208 and slab 204. The strip208, comprising doped semiconductor, provides electrical connectionwhile separating the metal electrode 210 from a substantial portion ofthe optical energy in the mode. An electric field may therefore beapplied to the desired light path while minimizing or reducing lightabsorption caused by strip electrode 210. Other transparent conductorscan alternatively be inserted between the strip electrode 210 and theperimeter portion of the slab 204. To further protect against lightabsorption by metal, portions or all of the strip and slab electrodes210, 212 may comprise conducting polysilicon, rather than metal.However, when adding high refractive material near the strip 208 andslab 204, however, care must be taken to ensure that the resonatorremains single-mode. Lower refractive index conducting material maytherefore be preferred.

[0116] As discussed above, the transparent strip 208 also serves toconfine the light to an optical path along the perimeter of the slab andtherefore defines the optical mode as is illustrated graphically inFIGS. 10A-10D. FIG. 10A portrays an perspective view of the disk-shapedslab 204 of FIGS. 2-4. FIG. 10B shows a cross-sectional view of slab204. The arrow within slab 204 indicates that light may propagatethroughout the interior of slab 204. This light corresponds to theoptical power associated with a number of different modes. As discussedabove, an evanescent field penetrates beyond the boundaries of the coreregion 204. FIG. 10C shows slab 204 together with the annular strip 208.The arrows in FIG. 10C illustrate the lateral spatial extent of theoptical mode supported within the slab 204 with the addition of thestrip 208 above the perimeter portions of the slab. As discussed above,light is confined to the portions underneath the strip 208. Confiningthe light to the perimeter region of the slab 204 may prevent multiplemodes from propagating within the slab. The dimensions of the strip 208and slab 204 as well as the respective indices of refraction and that ofthe surrounding cladding determine what modes are supported. Preferably,these parameters are selected to support single mode propagation withinthe resonant cavity 106.

[0117]FIG. 10D depicts the insulating layer 206 between theannular-shaped strip 208 and the disk shaped slab 204. As shown, thestrip 208 continues to provide confinement of the light in the peripheryof the disk shaped slab 204 despite the presence of the insulating layer206, if this layer is sufficiently thin.

[0118] As discuss above, the slab electrode 212 may be electricallycontacted with the slab 204 via an ohmic contact between slab electrodeand a central doped region 214 of slab. The doped region 214 of diskshaped slab 204 provides two additional advantages. As illustrated inFIG. 10D, this doped region 214 is preferably substantially located atthe center of the circular shaped slab 204 and the dopant added to thedoped region 214 preferably strongly absorbs light. Because thisabsorption is provided in the central portion of the resonant cavity204, light from the perimeter portion of the slab 204 that propagates tothe doped central region 214 is preferentially absorbed. The dopantadded to the doped region 214 also preferably lowers the refractiveindex of the central region of the slab 204, thus enhancing confinementof light within the periphery of the slab. As a result of these twoeffects, the doped region 214, like the strip 208 discussed above,promotes confinement of light to the perimeter portion of the resonantcavity 204. These design features can be used to prevent the higherorder modes from propagating within the slab such that substantially allthe optical power can be concentrated into the one optical modetraveling around the perimeter of the resonant cavity 204.

[0119] The remainder of the slab 204 including regions beneath the strip208 may also be doped p or n type so that the semiconductor slab isconducting. The dopant is at higher concentration at the center of theslab 204 to quench modes in that region. Depending upon the doping andother geometrical considerations, a positive or negative voltage may beapplied between the strip and slab electrodes 210, 212 in order tomodulate the refractive index of the optical path in the resonantcavity.

[0120]FIG. 11 shows another preferred resonant cavity 106 that allowsfor modulation of the free carriers in a resonant cavity through thefield effect. This structure includes a substrate 600 analogous to thesubstrates 200, 300 of the earlier embodiments, on top of which is alower cladding layer 602 like the layers 202, 302 in the embodimentsdescribed with reference to FIGS. 2-4 and FIGS. 5-7. This resonantcavity further comprises a disk-shaped slab 604, preferably comprisingsingle crystal silicon. Although this slab 604 is disk shaped, as inFIGS. 2-4, it may have other shapes and may, for example, be annular, asin FIGS. 5-7. An annular strip 608 is disposed over the disk-shaped slab604 along a perimeter region of the slab. First and second (strip andslab) electrodes 610, 612, analogous to the electrodes 210, 212, 310,312 of the earlier embodiments, may be used to apply an electric fieldinto the perimeter region of the slab 604. (A doped region, omitted fromFIG. 11 for clarity, is preferably included to create an ohmic contactbetween the slab electrode 612 and the slab 604.) An insulating layer606 covers the slab 604. This resonant cavity 106 further includes afirst thin insulating transition layer 616 that separates the strip 604from the slab 604 in a manner analogous to the transition regions 216,316 discussed above.

[0121] The resonant cavity 106 shown in FIG. 11, however, includes asecond thin insulating layer 626 on the strip layer 608. A gate 618 isformed over the second thin insulating layer 626. This gate 618 may beannular in shape to match the annular strip 604 layer below. This gatelayer 618, however, preferably has a width smaller than that of thestrip layer 604, that is, the difference between the outer and innerdiameter of the annular strip is greater than the difference between theouter and inner diameters of the annular gate layer. The first (gate)electrode 610 is connected to the gate 618. The second (slab) electrode612 is connected to the slab 604. The strip 608 preferably comprised ofpolysilicon and may alternatively be comprised of crystalline silicon.Other materials that are preferably conductive and have a highrefractive index in comparison to the surrounding insulating layer 606may also be used. The gate 618 also may be comprised of polysilicon orsingle crystal silicon. Other preferably conductive materials may beemployed as well. Materials having a lower refractive index than that ofthe strip 608 will be less likely to alter the shape of the optical modein the strip and slab 604.

[0122] The strip 608 serves to confine light laterally within the slab604 in a region below the strip and in proximity thereto. Thus, opticalpower is distributed in this region in the slab 604 as well as withinthe strip 608 and the thin insulating region 616 therebetween asdescribed above. This embodiment provides the advantage that the fieldeffect created by applying a voltage between the first and second (gateand slab) electrodes 610, 612 will may influence not only the freecarrier distribution in the slab 604, also the free carrier distributionin the strip 608. In the “whispering gallery” mode, there is significantoptical power both in the perimeter region of the slab 604 and in theannular strip 608. This design allows for the variable control of therefractive index in both of these locations. The free carrier densitybeneath the first thin insulating transition layer 616 between the strip608 and the slab 608 can be controlled as described above. In addition,with the configuration shown in FIG. 11, the concentration of freecarriers beneath the second thin insulating transition layer 626 and inthe strip 608 can be selectively altered.

[0123] The electron concentration can be controlled in the strip 608substantially independently, by electroding the gate 618 and the strip618 instead of the gate and the slab 608. A stronger affect on theelectron density below the gate 618 may be achieved in this manner. Inthis specific configuration the voltage is across the gate 618 and thestrip 608 and not across the strip and the slab 604. Otherconfigurations can be employed to yield different results. For example,two voltage sources can be utilized to provide independent variablecontrol of the carriers within the slab 604 and those within the strip608. One voltage source can establish a field across the first thininsulating transition layer 616 and another supply can induce a fieldthrough the second thin insulating transition layer 626.

[0124] As shown in FIG. 11, the width of the gate layer 618 ispreferably smaller than the width of the strip 608. This reduced widthis intended to reduce perturbation of the shape of the resonator modedue to the gate 618, which may comprise silicon, and have asubstantially similar refractive index as the strip 608, which may alsocomprise silicon. As discussed above, however, the strip 608 need not bealigned with the outer edge of the disk shaped slab 604 and the slab mayextend well beyond the strip. Moreover, the slab 604 need not bedisk-shaped an may comprise a wide planar sheet or layer ofsemiconductor material. In addition, the gate 618 need not be alignedwith the outer edge of the annular strip 608.

[0125] The resonant optical cavity 106 shown in FIGS. 5-7 operatessimilarly to the optical cavity of FIGS. 2-4. Light traveling on aclosed path within the annular resonant cavity 106 can interfereconstructively or destructively depending upon the relationship betweenthe optical path length of the closed path and the wavelength of thelight. Light traveling on paths for which the total optical path lengthis an even number of half-wavelengths will experience constructiveinterference; light traveling on paths for which the total path lengthis an odd number of half-wavelengths will experience destructiveinterference. Because of this phenomenon, the structure forms a resonantcavity 106 that resonates at certain frequencies.

[0126] Variable control of the resonant frequency of the opticalresonant cavity 106 may be achieved by changing the effective index ofrefraction of the material in the annular guiding region. The density offree carriers in a region of the resonant cavity may be changed via thefield effect by applying a potential difference between the strip andslab electrodes 310, 312. Applying the appropriate voltage between theelectrodes 310, 312 creates an electric field causing either electronsor holes to depleted or accumulated at the top surface of the slab 304beneath the annular strip 308. These electrons or holes cannot freelyflow between the strip 308 and the slab 304 because of the existence ofthe insulating layer 316 between slab 304 and the strip 308. In thismanner the resonant frequency of the cavity can be tuned.

[0127] As discussed above, the resonant cavity 106 preferably supports asingle optical mode, such as the “whispering gallery” mode. Thisobjective may be accomplished by having the width of the guiding regionsufficiently narrow that only one optical mode is guided. In onepreferred design, the outer edge of the slab 304 can be used to provideconfinement. In addition, the slab 304, in this case annular shaped, maybe sufficiently narrow, i.e., the distance between the outer diameterand the inner diameter is sufficiently small, to prevent other modesfrom existing. Strong confinement can also be a provided by thesufficiently narrow annular shaped strip 308 disposed above the slab304.

[0128] As discussed above, to facilitate application of an electricfield within the slab 304, the slab electrode 312 is electricallyconnected to the surface of the slab. Preferably, an ohmic contact isformed by appropriately doping the contact region of the slab 304. Thishigh concentration of dopant which may, for example, be concentratedtoward the inner portion of the annular slab 304, may also assist inconfining the optical mode to a localized region on the outer portion ofthe slab 304. The dopant may reduce the refractive index in the highlydoped region thereby enhancing confinement or may absorb optical energyoutside the guiding region.

[0129] The dimension of the slab 304 and strip 308 in large part alongwith the material the associated refractive index, define what modes aresupported by the waveguide structure. These dimensions depend paritallyon the wavelength of light for which the resonant cavity 106 is designedto operate. Various embodiments may be designed for light having awavelength between about 1.3 and 1.6 micrometers. However, thesestructures are not to be limited to any particular wavelength orwavelength range and may be designed for microwave, infrared, visible,and ultraviolet wavelengths.

[0130] The thickness of the insulating transition layer disposed betweenthe strip 208, 308, and the slab 204, 304 depends on the materials andon the voltage to be applied to effectuate the desired index change. Thewaveguide structures may be appropriately configured to suit thespecific voltage range and index change.

[0131] As discussed above, these structures may be fabricated fromsemiconductor material such as single crystal silicon and polysilicon aswell as dielectrics such as silicion dioxide. Other materials may alsobe employed. Moreover, other semiconductor and dielectrics may also beemployed. In addition, various metals may be employed to form conductivepathways although non-metal conductors are also suitable and may bepreferred in certain circumstances.

[0132] In addition, although the optical path is toward the outermostregions of the slab 204, the optical path need not be limited to thislocation on the slab. In other embodiments, for example, the slab may belarger and may not even be circular. A closed optical path, circular ornon-circular, may be provided by, for example, strip loading or byridges or ribs positioned elsewhere than on the outermost edges of theslab. However, more compact designs might be those depicted in FIGS.2-7.

[0133] As discussed above, the waveguide structures are not limited toany particular type, such as a strip loaded waveguide having arelatively low index transition layer. Rib or ridge, slab, channel, andconventional strip loaded waveguide designs may be employed. Forexample, tunable resonant cavity can be formed from a ridge waveguidestructure comprising semiconductor. A thin insulating layer can beformed over the ridge and metallization can be deposited on the thininsulator to form an electrode. The semiconductor can also be electrodedand a voltage applied between the preferably doped semiconductor ridgeand the metallization atop the thin insulating layer. The electric fieldthrough the thin insulating layer will induce the accumulation ordepletion of free carriers in the semiconductor ridge altering itsrefractive index. In this manner, the index of refraction of a ridgewaveguide can be manipulated.

[0134] Similar designs can be implemented for slab, channel, and stripwaveguides comprising semiconductor. Namely, a thin insulating layer canbe formed over these waveguides and metallization can be deposited onthe thin insulator to create an electrode. Applying a voltage to themetallization and preferably the doped semiconductor may cause electronsor holes to be depleted or accumulated in the semiconductor altering therefractive index therein.

[0135] In these designs, the metallization within close proximity to thesemiconductor waveguide may interact with the optical mode absorbingoptical energy and introducing attenuation. Crystal or polycrystallinesilicon can be substituted as an electrode material, however, the indexof this material may be sufficiently high and may perturb the opticalmode, depending on the particular design. The shape of electrode maytherefore be specifically shaped to yield the desired result.

[0136] Other configurations are considered possible and may be moresuitable for specific applications. For example, photonic bandgapcrystal waveguides may be used, however, the dependency of the index ofrefraction on carrier density may depend on a number of factors.Nevertheless, the usable waveguide structures are not to be limited tothose described herein and may include types yet to be discovered ordeveloped.

[0137] D. Operation of the Optical Switching Apparatus

[0138] The operation of an optical switching apparatus 104 incorporatingthe resonant optical cavity 106 of FIGS. 2-4 will now be described.Associated with the optical switching apparatus 104 is an opticalsource, preferably a laser. This light source is preferably a continuouswave (CW) source, although the operation of the switching apparatus 104is not so limited. The light output has a characteristic wavelength andoptical frequency determined by the optical source. The resonant opticalcavity 106 is designed to resonate at a frequency either at, or offsetfrom, the optical frequency of the light source.

[0139] The resonant cavity 106 may comprise resonators such as thosedescribed or may comprise another type of resonant cavity. The followingdiscussion will assume for illustrative purposes that the resonantcavity 106 comprises the configuration of FIGS. 2-4. It should be isunderstood, however, that the discussion applies to other resonantcavities as well.

[0140] As described above, the resonant frequency of the opticalresonant cavity is determined by the length of the optical path aroundthe circular guiding region of the slab 204 and the effective refractiveindex in this optical path. The dimensions and material of the resonantcavity 106 should be selected to create a resonant frequency close tothe optical frequency of the light source. Due to manufacturingtolerances, however, the resonant frequency of a particular resonantcavity is difficult to produce with sufficient precision. As such, aftermanufacturing, the resonant frequency of a particular optical resonantcavity may be adjusted, for example, through thermal tuning.

[0141] Thermal tuning refers to the manipulation of the resonantfrequency of the cavity through control of the temperature of the cavitymaterial. This tuning may be accomplished by thermally coupling atemperature control unit to the resonant cavity 106 that allows thetemperature of a portion, or all, of the resonant cavity 106 to beadjusted. A Peltier heating/cooling system, for example, may be inthermal contact with the resonant cavity 106. Resistive or other heatingor cooling mechanisms may be employed as well to control the temperatureof the waveguide structure.

[0142] Raising the temperature of the resonant cavity 106 alters theresonant frequency of the resonant cavity in two ways. First, thermalexpansion of the disk-shaped slab 204 increases its diameter and thepath length around the perimeter. The resonant frequency of a particularmode of the cavity can thereby be decreased. Second, the increase intemperature increases the number of free carriers in the resonant cavity106, decreasing the refractive index of the resonant cavity, and thusincreasing the resonant frequency. Because the latter effect is muchstronger than the former, increasing the temperature of the resonantcavity 106 increases the resonant frequency. Once an optical resonantcavity has been manufactured and tested, its temperature may be raised(via heating) or lowered (via cooling), as needed, to tune the resonantfrequency to the optimal frequency for a particular application.

[0143] Controlling the temperature of the resonant cavity can beemployed instead of or in addition to applying an electric field toalter the free carrier density in waveguide structures and adjust ormodulate the index of refraction. Thermal tuning, however, may not be asfast as tuning by using the field effect. In certain embodiments,thermal tuning will be used to adjust the operating point of for theresonant frequency of a resonant cavity and the field effect will beemployed to rapidly modulate the tuning.

[0144] In operation, an optical input from the optical source ispropagated down the first waveguide 100 shown in FIG. 1. Because therefractive index of the first waveguide 100 is much larger than therefractive index of the cladding region 108, the waveguide 100propagates light in a guided fashion, as discussed previously.

[0145] When it is desired that the input signal remain in the firstwaveguide 100 (i.e., to produce an output signal from the firstwaveguide 100), the optical resonant cavity 106 within the opticalswitching apparatus 104 is set to a state where the resonant frequencyis offset from the optical frequency of the light source. Light of thisoptical frequency traveling on a closed path within the annular resonantcavity 106 interferes destructively therein. Accordingly, resonance isnot achieved at this wavelength and light is not output from theresonant cavity 106, which as a result blocks coupling between the firstand second waveguides 100, 102. In such a state, the light from thelight source continues propagating down the first waveguide 100 withouttransferring any substantial amount of optical energy into the secondwaveguide 102.

[0146] Conversely, when it is desired that the input light switch to thesecond waveguide 102 (i.e., produce an output signal from the secondwaveguide 102), the optical resonant cavity 106 within the opticalswitching apparatus 104 is set to a state where the resonant frequencysubstantially matches the optical frequency of the light source. Asdiscussed above, this frequency shifting may be accomplishedelectronically by modifying the voltage between the first and secondelectrodes 210, 212, shown in FIGS. 2-4. The thermal state, i.e., thetemperature, of the resonant cavity 106 can also be changed so as toalter the free carrier concentration within the guiding region of theresonant.

[0147] The strength of the coupling between the first waveguide 100 andthe optical resonant cavity 106 will depend upon the spacing A betweenthe waveguide and the resonator as well as the dimensions and materialsof the first waveguide 100 and the resonant cavity. Light from the lightsource traveling on the closed path within the annular resonant cavity106 interferes constructively therein. Accordingly, resonance isachieved. The cavity is filled with a high intensity electro-magneticfield. Some of this electromagnetic energy is transferred from theresonant cavity 106 into the second waveguide 102 and is outputtherefrom. Accordingly, when the optical resonant cavity 106 is tuned tothe optical frequency of the light source, the light propagating in thefirst waveguide 100 can be strongly coupled or “dropped” into the secondwaveguide 102. The proportion of optical energy within the firstwaveguide 100 that is transferred to the second waveguide 102 depends ona number of factors such as the coupling efficiencies between the firstwaveguide and the resonant cavity 106, and between the resonant cavityand the second waveguide, as well as the absorption and scatteringlosses within the optical resonator.

[0148] Preferably, the relationship of the first waveguide 100 withrespect to that of the optical resonant cavity 106 is designed so thatthe all of the optical energy from the first waveguide 100 istransmitted into the resonant cavity 106 when on resonance. Under thiscondition, known as “critical coupling,” light coupled back from theresonator into the first waveguide 100 destructively interferes with theremaining light present in the first waveguide 100. As such, no energyis output from the first waveguide 100. Instead, the optical power isfully transmitted into the resonant cavity 106, where it is lost throughtwo sources: 1) scattering/absorption in the resonant cavity 204; and 2)coupling of light into the second waveguide 102.

[0149] The power coupled into the second waveguide 102 will necessarilybe lower than the power introduced in the first waveguide 100, due tothe scattering/absorption within the resonant cavity. However, themagnitude of the electromagnetic field strength in the second waveguide102 (i.e., the output signal) will be roughly proportional to theelectromagnetic field strength in the first waveguide 100 (i.e., theinput signal), with the proportionality constant determined by the sizesof the losses in the cavity.

[0150] There are different ways in which the system may be configured totransmit the optical power to either the first waveguide 100 or secondwaveguide 102. For example, the system may be designed so that applyingthe modulation voltage increases the frequency of the resonant cavity106 to the optical frequency from a lower starting frequency.Conversely, the system may be designed so that applying the modulationvoltage decreases the frequency of the resonant cavity 106 to theoptical frequency from a higher starting frequency. In anotherembodiment, the resonant frequency may match the optical frequency whenthere is no applied electric field in the cavity.

[0151] As discussed above, the introduction or depletion of freecarriers in a region has an effect on the absorption of lightpropagating through that region. As the free carrier density in guidingregion of the resonant cavity 106 changes, the degree to which light isabsorbed while passing through this region also changes. As such, whenthe carrier density along the optical path in the resonator is free ofcarriers, or depleted, the absorption of light will be less than whenfree carriers have been accumulated. Conversely, when free carriers areinjected along the optical path in the resonator, the absorption oflight in the resonator is increased. This relationship holds true formost waveguide structures. Photonic crystal band gap waveguides may varydifferently.

[0152] As is well known, resonant systems may be characterized by adimensionless “quality factor” commonly referred as Q, where:$Q = \frac{f_{0}}{\Delta \quad f}$

[0153] where ƒ₀ is the resonant frequency of the resonator and Δƒ is thefull-width at half-maximum of the power spectrum of the resonatorsystem. The Q of a resonant cavity determines the field strength withinthe cavity. There is an inverse relationship between absorption in theresonant system and Q. As such, generally when the carrier density alongthe optical path in the resonator is free of carriers, or depleted, Q isincreased. On the other hand, when free carriers are injected along theoptical path in the resonator, Q is decreased.

[0154] The relationship between free carrier density and Q allows thecoefficient Q to be tuned simultaneously with the tuning of the resonantfrequency. FIG. 12 is a plot of power spectra for a resonator in twodifferent states. In the first state, the resonator is tuned to resonantfrequency, ƒ₁ by accumulating carriers. This accumulation of carriersalso results in absorption and a lower quality factor, Q₁. In the secondstate, the resonator is tuned to a higher another resonant frequency,ƒ₂, by depleting carriers. With less carriers and less absorption, thequality factor, Q₂ is lower. By selecting the size and composition ofthe resonant cavity, together with any thermal tuning, a particular Qvalue can be achieved at a desired frequency. For example, two resonantcavities of different dimensions can be designed to having identicalresonant frequencies and different Q values because one of them isthermally tuned to include more free carriers. This flexibility isadvantageous when the cavity is to be used as a filter where control ofQ is desirable. As discussed above, tuning can be alternatively achievedby applying an electric field to accumulate or deplete carriers as well.Thermal and electrical tuning can be utilized together as well.

[0155] When operated as an optical switch, it is advantageous that thedensity of free carriers be reduced within the resonant cavity 106 whenthe optical switch is coupling light from the first waveguide 100 to thesecond waveguide 102. With lower amounts of free carriers, losses due toabsorption can be reduced. Furthermore, it is advantageous that thedensity of free carriers be increased within the resonant cavity 106when the optical switch is not coupling light from the first waveguide100. In this latter case, it is desirable that the light continuepropagating along the first waveguide 100, with no power provided to thesecond waveguide 102, i.e., with reduced or negligible cross-talkbetween the two waveguides. By increasing absorption, losses in theresonant cavity 106 can be enhanced, and reflections from the resonantcavity back to the first waveguide 102 can be curtailed.

[0156] These conditions are accomplished by a configuration thatdecreases the number of carriers in the resonant cavity 106 when tuningthe resonant frequency of the cavity to match the optical frequency ofthe input signal. For example, the system may be thermally tuned to havea resonant frequency matching the optical frequency in the absence of anapplied voltage between the first and second electrodes 210, 212. Underthese conditions, light will be dropped down from the first waveguide100 to the second waveguide 102 through the resonant cavity 106 whenthere is no applied voltage from the voltage source 114. Applying avoltage then shifts the resonant frequency of the cavity away from theoptical frequency, removing the coupling between the first waveguide 100and the resonant cavity 106, thus switching the output signal to thefirst waveguide. Advantageously, carrier concentration and theconsequent absorption is less in the state where energy is dropped fromthe first waveguide to the second waveguide, than when the resonator isdetuned with the application of the voltage.

[0157] The relationship between the output signal on the first waveguide100 (“Output 1”) and on the second waveguide 102 (“Output 2”) 112 forsuch a system is shown in FIG. 13. The modulation of the voltage appliedbetween first and second electrodes 110 is shown at the top. A voltageof zero (i.e., no applied field effect in the resonant cavity 204),results in a HIGH output in the second waveguide 102 and a LOW output inthe first waveguide 100. Conversely, when a modulation voltage isapplied (inducing the field effect in the resonant cavity 204), theoutput in the second waveguide 102 drops to LOW, and the output in thefirst waveguide 100 changes to HIGH.

[0158] Alternatively, the system may be designed to create a mismatchbetween the resonant frequency of the cavity and the carrier frequencyof the optical source in the absence of an applied voltage. For such asystem, application of the voltage shifts the cavity resonant frequencyto match the carrier frequency, causing the output signal to drop to thesecond waveguide 102. For this system, the outputs on the two waveguideswould be reversed from what is portrayed in FIG. 13.

[0159] E. Modulating the Coupling Coefficient Between Optical Structures

[0160] In some applications, it is advantageous to modulate the couplingbetween two optical waveguide structures. The coupling may, for example,be controlled so as to preferentially allow, or preclude, propagation oflight from one optical waveguide structure to the other. If the opticalstructures are comprised of semiconductor, such as silicon, modulationof the free carriers in one or both of the semiconductor waveguides maybe used to manipulate the refractive index, thus altering theconfinement of optical modes therein and the coupling between thestructures.

[0161]FIG. 14A shows a pair of waveguides arranged to form a directionalcoupler. The waveguides include a core and cladding regions. The core issurrounded by the cladding material, not shown in FIG. 14, and as such,these waveguides may be considered channel waveguides although thewaveguides should not be limited to any particular type. The corepreferably has an index of refraction higher than that of the cladding.The core preferably comprises semiconductor such as for example singlecrystal silicon. The core may alternatively comprise polycrystalline aswell as other semiconductors which are preferably substantiallytransparent at the wavelength region of interest. In various embodimentsthese semiconductors are preferably doped. The surrounding claddingmaterial preferably comprises a dielectric such as silicon dioxide,although other relatively low refractive index insulators, such as air,may be used. The cladding in some cases may provide electricalinsulation between the two waveguide structures. FIG. 14B shows across-sectional view of a coupling region within of directional couplershown in FIG. 14A taken through the line 14B-14B.

[0162] Depending upon the relative refractive index of the core andcladding, the evanescent field associated with light propagating throughthe waveguides will extend beyond the core different amounts. With lessconfinement, the evanescent field will continue farther outside thecore. Since the waveguides are within close proximity to each other, thespatial modes begin to overlap facilitating the transfer of opticalenergy therebetween. This effect is illustrated graphically in FIGS.15A-15C, which shows three plots of intensity versus distance along theline 15-15 shown in FIG. 14B. FIG. 15A shows the intensity for lightpropagating through each waveguide when the refractive index of the coreregion of each waveguide has a particular value, n₁. FIG. 15B shows theintensity when the refractive index the core region of each waveguide islowered, i.e., n₂<n₁. As shown in the figure, the lateral spatial extentof the evanescent field extends out further from the within thewaveguide, increasing the overlap between the two fields. FIG. 15C showsthe intensity when the refractive index of the core region eachwaveguide is raised such that n₃>n₁. In this case, the lateral extent ofthe evanescent fields shrinks, decreasing the overlap between the twofields.

[0163] The overlap between the two fields may be quantified bycalculating an “overlap integral” that provides a measure of thestrength of the coupling between the two waveguides which depends on theshape of the optical modes therein. The overlap integral may be used todetermine a “coupling coefficient,” wherein the stronger the couplingbetween the structures, the higher the associated coupling coefficient.Inspection of FIGS. 15A-15C reveals that the waveguides associated withFIG. 15B have a larger coupling coefficient than the waveguidesassociated with FIG. 15C.

[0164] As discussed previously, the refractive index of a semiconductor,such as silicon, may be varied by altering the distribution of freecarriers. Likewise, the coupling coefficient associated with a pair ofclosely spaced optical waveguide structures comprised of semiconductormay be manipulated by altering the distribution of free carriers in oneor both of the structures.

[0165]FIG. 16 depicts a configuration for modulating the couplingcoefficient associated with the directional coupler of FIG. 14. As shownin FIG. 16, the first waveguide 700 and the second waveguide 702 may beelectrically connected to a voltage source 708 via a first electrode 704and second electrode 706, respectively.

[0166] Applying a voltage between the first and second electrodes 704,706 results in the capacitive storage of free carriers within the coreregion of each waveguide 700, 702. The capacitance between the twowaveguides permits storage of opposite charge on each waveguide, whichcan be employed to alter the refractive index of the waveguides. Theapplied voltage may for example induce free carriers of opposite sign ineach waveguide 700, 702. These carriers may be concentrated in the coreadjacent the cladding between the two waveguides 700, 702 but thischarge preferably extends through portions of the core where the opticalmode is distributed. The insulating cladding material prevents thecarriers from freely flowing between the two waveguides 700, 702.Application of the voltage creates an electric field, which may induceelectrons to accumulate in the first waveguide 700, and holes toaccumulate in the second waveguide 702, and vice versa, depending, forexample on the doping. Thus, the voltage may be employed to increase ordecrease the effective refractive indexes of either of the waveguides700, 702. The particular affect of the applied voltage depends on thedoping. For example, if the first and second waveguides 700, 702 are nand p doped, respectively, an applied voltage may be used to increasethe refractive index of both waveguides. Confinement is strengthened,shrinking the associated evanescent fields and thereby decreasing thecoupling coefficient. With a sufficiently large voltage, coupling withinthe waveguides can be reduced to a substantially zero. Changing the signof the applied voltage in this scenario decreases the refractive indexof both waveguides. The confinement is weakened, broadening the range ofthe associated evanescent fields and thereby increasing the couplingcoefficient such that the waveguides are more strongly coupled.

[0167] If the coupling regions of the waveguides 700, 702 are eachn-type or p-type, applying a similar voltage to each will affect eachwaveguide differently: raising the refractive index in one waveguidewhile lowering the refractive index in the other waveguide. With thiselectrical configuration, however, the refractive index in eachwaveguide is modulated substantially simultaneously.

[0168]FIG. 17 shows another configuration for modulating the couplingcoefficient associated with the directional coupler depicted in FIG. 14.In this design, the capacitance between the two waveguides and anotherclose-by structure permits storage of charge on the waveguides. Asshown, the first waveguide 720 and the second waveguide 722 areelectrically connected to separate voltage sources 727, 728 via a firstelectrode 724 and second electrode 726, respectively. Applying voltagesV₁ and V₂ between the waveguides 720, 722 and a nearby structure, suchas the substrate 729 of an SOI wafer, results in the capacitive storageof free carriers of the coupling region of each waveguide. Dependingupon the sign of the applied voltages V₁ and V₂ and the doping of thewaveguides (i.e., n-type or p-type), the refractive indexes in thecoupling regions of the waveguides 700, 702 may be independentlyincreased or decreased. As such, the confinement within the two guidesand the resulting coupling therebetween can be modulated as desired. Thecoupling coefficient will also vary accordingly.

[0169]FIG. 18 shows another configuration for modulating the couplingcoefficient associated with the directional coupler depicted in FIG. 14.As shown in FIG. 18, the first and second waveguides 730, 735 mayinclude both a p-type region 731, 736 and an n-type region 732, 737 soas to form p-n junctions. These waveguides are independentlyelectrically connected to voltage sources 734, 739.

[0170] The free carrier distribution associated with p-n junctionsexposed to an applied voltage is very well known. In the absence of anapplied field, some of the free electrons in the n-type region 732, 737diffuse across the junction and combine with holes in the p-type region731, 736. The region in which electrons and holes combine form adepletion region 733, 738 lacking in free electrons and holes. Theapplication of a voltage across the p-n junction either expands orcontracts the size of the depletion region 733, 738, depending upon thesign of the applied voltage. Forward biasing the p-n junction shrinksthe depletion region 733, 738 and, if the voltage exceeds about aspecific threshold amount, e.g., 0.5 volts, depending on the design, asubstantial electrical current is created across the junction. Reversebiasing the p-n junction expands the depletion region 733, 738 andresults in essentially no electrical current unless the applied voltageexceeds a threshold “breakdown voltage” of the junction. When thebreakdown voltage is exceeded, reverse bias creates a large electricalcurrent across the junction. Thus, for low power operation, thisstructure is preferably operated under reverse bias conditions below thebreakdown voltage when the current through the junction is not as highas in the other modes of operation.

[0171] Applying a reverse bias to the p-n junction of one of thewaveguides 730, 735 depletes free carriers and increases the refractiveindex in the waveguide thereby enhancing confinement and correspondinglydecreasing the evanescent field. With each p-n junction electricallyconnected to a different voltages, respective voltages can be appliedindependently to the first and second waveguides 730, 735. Thus, thecoupling coefficient can be modulated as desired. Various otherconfiguration are possible such as those discussed above, for example,wherein a single voltage source is electrically connected to bothwaveguides 730, 735 and/or electrical connections are made to a commonelectrical plane.

[0172]FIG. 19 illustrated how such coupling between two strip loadedwaveguide portions having insulating transition layers can be controlledin a directional coupler. The technique for modulating the couplingcoefficient between two waveguides portions is similar to that describedabove. As illustrated in FIG. 19, to form the directional coupler aplanar slab 744 is disposed on top of a cladding layer 742 formed on asubstrate 740. As shown, air may surround top portions of the slab 744creating a total internal reflection boundary at the slab/air interface.The slab 744 preferably comprises a material having a higher refractiveindex than the lower cladding layer 742 to, along with the slab/airinterface, provide the vertical confinement. Preferably, the slab 744comprises semiconductor which is doped and the cladding layer 742comprise a dielectric. In one preferred embodiment, the planar waveguidecomprises silicon, e.g., single crystal silicon, and the insulatinglayer comprises silicon dioxide, which forming the conventional SOIstructure discussed previously.

[0173] Light is confined horizontally within the planar waveguide 744along two distinct paths defined by first and second strips 748, 749,each extend longitudinally to guide the light along curvilinear pathssuch as shown in FIG. 14. As discussed earlier, the high refractiveindex strips 748, 749, have the effect of substantially confining thelight to the regions beneath them. These elongated strips 748, 749 arepreferably comprised of polysilicon. Alternatively, they may comprisesingle crystal silicon. Other material may be used as well, as describedabove.

[0174] This structure further includes insulating transition layers 746,747 between the strips 748, 749 and the substantially planar slab 744 soas to allow for field effect manipulation of the free carriers in thewaveguides. A voltage can be applied through the first strip 748 via afirst strip electrode 750 electrically connected to a first variablevoltage source 760. A voltage can be applied through the second strip749 via a second strip electrode 752 electrically connected to a secondvariable voltage source 762. Each voltage source is preferablyelectrically connected to the surface of the substantially planar slab744 via leads 751, 753 that form ohmic contacts with a doped region(omitted from FIG. 19 for clarity) on the slab 744.

[0175] As discussed previously, the field effect may be used to controlthe distribution of free carriers in a semiconductor, such as thesubstantially planar slab 744. Thus, by applying voltages V₁, V₂ acrossthe insulating layers 748, 747, it is possible to increase or decreasethe refractive index in portions of the slab 744 underneath the strips748, 749. Thus, the shape of the optical mode within the guiding regioncan be control, and the coupling coefficient can be modulated asdesired.

[0176]FIGS. 20A and 20B illustrated how the coupling coefficient betweenan elongated waveguide and a resonant cavity can be variably controlled.FIG. 20A is a top view of the elongated waveguide and resonant cavity.FIG. 20B is a cross-sectional view along the line 20B-20B in FIG. 20A.As shown, the waveguide and resonant cavity comprise elongated anddisk-shaped slabs 854, 804, respectively, disposed atop a lower claddinglayer 802 formed on a substrate 800. An insulating layer 806 is formedover the slabs 854, 804 to provide an upper cladding layer as well as toprovide electrical insulation for conductive pathways in the structure.The elongated and disk-shaped slabs 854, 804 preferably comprisematerial having an index of refraction higher than that of the upper andlower cladding layers 806, 802 to provide vertical confinement. Theslabs 854, 804 preferably comprise semiconductor, which may be doped,and lower cladding layer comprises dielectric material. Morespecifically, the slabs 854 and 804 preferably comprise single crystalsilicon and the lower cladding layer 802 preferably comprises silicondioxide, forming the SOI structure discussed previously.

[0177] An elongated strip 858 is formed over the waveguide slab 854 andan annular shaped strip 808 is disposed over the disk-shaped slab 804.Light is confined horizontally within the slabs 854, 804 by therespective strips 808, 804. The geometry of the structures are such thatlight will propagate longitudinally along the elongated slab 854 andaround a circular path around the periphery of the disk-shaped slab 804.The strips 808, 858 are preferably comprised of polysilicon or singlecrystal silicon. Alternatively, they may comprise other materials asdiscussed above.

[0178] Thin insulating transition layers 816, 856 separate the strips858, 808 from the slabs 854, 804 in order to allow for field effectmanipulation of the free carriers in the elongated waveguide and theresonator. A voltage can be provided through the strip 808 associatedwith the resonator via an electrode 810 electrically connected to afirst voltage source (not shown in FIG. 20). A voltage can be providedthrough the strip 858 associated with the elongated waveguide via anelectrode 860 electrically connected to a second voltage source (alsonot shown). The first voltage source also is preferably electricallyconnected to the top surface of the disk-shaped slab 804 via anelectrode 812 that forms an ohmic contact with a doped region 814 of thedisk-shaped slab 804. The second voltage source is preferablyelectrically connected to the top surface the elongated slab 854 via anelectrode 852 that forms an ohmic contact with a doped region of thewaveguide 854.

[0179] By applying a voltages across the annular strip 808 and thedisk-shaped slab 804, the refractive index underneath the annular stripmay be increased or decreased. Likewise, by applying a voltage acrosselongated waveguide 854 and the elongated strip 858, the refractiveindex underneath the elongated strip may be increased or decreased.Thus, the coupling coefficient between the elongated waveguide and theresonant cavity 804 can be modulated as desired. Modulation of thecoupling coefficient can be implemented in addition to tuning theresonant frequency of the resonator cavity.

[0180] In some applications, it is advantageous to maintain a constantcoupling coefficient (e.g., to maintain critical coupling) between awaveguide and a resonant cavity while manipulating the resonantfrequency of the resonant cavity. The embodiment of FIGS. 20A and 20Ballows for such flexibility. In particular, as the refractive index inthe resonant cavity is altered in order to shift the resonant frequency,an unwanted consequence may be a shift in the coupling coefficientcaused by the shrinking or expanding of the evanescent field in theresonant cavity. This undesirable consequence can be offset bymanipulating the refractive index of the waveguide to maintain the sameoverlap integral between the fields in the waveguide and the resonantcavity. Thus, if tuning of the resonator shrinks the field in theresonator, the waveguide can be simultaneously tuned to expand the fieldin the waveguide so as to maintain a constant overlap integral.Conversely, if tuning of the resonator extends the field in theresonator, the waveguide can be simultaneously tuned to shrink the fieldin the waveguide so as to maintain a constant overlap integral. Bytuning the waveguide in step with tuning the resonant cavity, a constantcoupling coefficient may be maintained. As discussed above, this featuremay be advantageous in maintaining critical coupling which requires thatthe coupling efficiency match the losses in the cavity. However, inother applications, the coupling coefficient may not need to beconstant.

[0181] The structures and techniques for varying the effective index ofrefraction within a waveguide by altering the distribution of freecarriers therein can be applied to waveguide gratings 900 such as theone shown in FIG. 21. The waveguide grating 900 comprises a channelwaveguide 902 on top of a cladding layer 904 disposed on a substrate906. The channel waveguide 902 preferably comprises semiconductor, andmore preferably comprises doped semiconductor. Also, the channelwaveguide 902 preferably has higher index of refraction than thecladding layer 904. In one preferred embodiment, the channel waveguide902 comprises crystal silicon (e.g., active crystal silicon) and thecladding layer 904 comprises a silicon dioxide layer (e.g., aburied-oxide layer) on a silicon substrate 906.

[0182] An insulating layer (not shown) or a plurality of such layers maycover the channel waveguide 902 on top as well as on one or more sides.The insulating layer preferably has a refractive index lower than therefractive index of the channel waveguide 902, so as to act as an uppercladding layer confining light within the channel waveguide. Theinsulating layer preferably comprises silicon dioxide, which has arefractive index substantially lower than the refractive index of singlecrystal silicon. The insulating material may comprise other materialssuch as for example silicon nitride and polymers, like polyimide. Theinsulating layer or layers also prevents unwanted flow of electricalcurrent between conducting elements of the device.

[0183] A plurality of strips or elongate members 908 are arranged overthe channel waveguide 902 to created a grating. In some preferredembodiments, the plurality of strips 908 comprise a material having anindex of refraction different from the insulating layer (not shown)formed on the channel waveguide 902 so as to perturb the effectiverefractive index of the channel at localized regions therein. Theplurality of strips 908 is disposed over but space apart from thechannel waveguide 902. Preferably, the strip material is substantiallyconductive and may comprise doped semiconductor. In certain preferredembodiments, the plurality of strips 908 comprise doped polysilicon orsingle crystal silicon, however, other different materials may be usedfor the strips 908.

[0184] The strip 908 is separated from the channel waveguide 902 by aninsulating layer 910 comprised of dielectric material to prevent theflow of current between the strip and the channel waveguide and tothereby facilitate carrier accumulation and depletion. This transitionlayer 910 preferably has sufficiently thickness such that the carriersdo not traverse this barrier either by tunneling or through defects suchas “pin hole” defects. Conversely, the thickness of this dielectriclayer 910 is preferably not so large as to require a large voltage to beapplied to the device to accumulate or deplete the desired amount ofcarriers. In one preferred embodiment, this transition layer 206comprises silicon dioxide.

[0185] As shown in FIG. 21, the waveguide grating 900 further includesstrip electrodes 912 electrically connected to the strips 908. Ohmiccontacts and silicide may be used to create suitable electricalconnections between the electrodes 912 and the strips, which preferablycomprise doped semiconductor. These strip electrodes 912 areelectrically connected to a voltage source 914. This voltage source 914may be an AC or DC voltage supply depending on the particularapplication. Some or all of the strip electrodes 912 may electricallyconnected together and to the voltage source 914. Alternatively, one ormore voltage sources can be electrically connected to individual orgroups of electrodes 912 associated with different strips 908. Thewaveguide grating 900 preferably includes one or more channel waveguideelectrode 916 electrically coupled to a surface of the channel waveguide902. Once again, ohmic contacts and silicide may be employed to producea low resistance electrical connection between the channel electrode 916and the channel waveguide 902. The strip and channel waveguideelectrodes 912, 916 preferably are comprised of metal, although one ofskill in the art would recognize that other materials, such as dopedpolysilicon, may be used. Other configurations for establishing anelectric field between the strips 908 and the channel waveguide 902 arepossible and should not be limited to the electrical arrangementillustrated in FIG. 21.

[0186] As indicated by the arrow 920, optical power propagatinglongitudinally within the channel waveguide 902 is guided therein. Asdescribed above, the effective index of refraction within the channelwaveguide 902 is higher than the surrounding cladding regions, e.g.,upper and lower claddings 904, and is accordingly confined laterallytherein. The strips 902, however, may perturb the effective index of thechannel waveguide 902. The periodic perturbation of the effective indexof refraction creates a grating that scatters the light within thechannel waveguide 902. If the strips 908 are appropriately placed, lightof the desired wavelength will be coupled out of the channel waveguide902 at a specific angle, θ, as illustrated by arrow 922. This angle, θ,is determined in part by the effective index of refraction within thechannel waveguide 902 as well as by the spacing of the grating. Theserelationships are governed by well known principles of Bragg diffractionset forth in the following equation:${\frac{2\pi}{\lambda}n_{eff}\sin \quad \theta} = \frac{{\pm m}\quad \pi}{\Lambda}$

[0187] where Λ is the grating spacing frequency (i.e., the inverse ofthe grating spacing), n_(eff) is the effective refractive index, m isthe diffraction order, and λ is the wavelength of the light.

[0188] The electrodes on the strips 908 and the channel waveguide 902facilitate application of a potential difference between the strips andchannel waveguide. As discussed above, by applying an electric field tothe channel waveguide 902 through the insulating layer 910, the localdistribution of carriers below the individual strips 908 can be adjustedand controlled. For example, applying a positive voltage to a strip 908formed over a p-type semiconductor channel waveguide 902 will induce anelectric field that will cause depletion of majority carriersimmediately below the strip. Under certain conditions, inversion may beproduced wherein negatively charged carriers are attracted to thedepletion region. In either case, the free carrier distribution can becontrolled and varied. By altering the concentration of free carriers,the localized effective index beneath the individual strips 908 can beadjusted as desired. Accordingly, the effective index beneath one ormore strips 908 can be increased or decreased by application of theappropriate voltage to the selected strip. In this manner, the scattercross-section of the particular “ruling” or strip 908 of the grating canbe varied and controlled yielding either increased or decreasesscattering and resultant output coupling. In addition, by altering theeffective index of refraction, n_(eff), the angle of output, θ, or thewavelength at a particular angle, θ, can be altered and controlled as isset forth in the Bragg equation referenced-above. Switchable couplingand tunable filtering can be implemented in this manner. Otherapplications of controlling the carrier distribution within the gratingcoupler are also possible and are not limited to those discussed above.Increasing the electron density may also result in elevated levels ofabsorption, which may theoretically be desirable in certainapplications, and conversely, depleting free carriers may reduceadsorption.

[0189] Other waveguide grating configurations are possible as well. Thewaveguide grating 900 may, for example, be implemented using waveguidesother than channel waveguides. Ridge or rib waveguides, slab, and striploaded waveguides with or without low index transition layers are a fewexemplary candidates but the structures and designs should not belimited to these. In addition, the waveguides may have different shapesand may be integrated together with different structures. Differentmaterials and dimensions may be used. Finally, the grating design mightbe otherwise and may be altered depending on the application. Forexample the “rulings” or strips 908 can be shaped differently and mayhave other than rectangular or square cross sections. These grating mayfor instance be blazed. Still other variations in design are consideredpossible. In addition, in the case where the grating waveguide 900comprises a strip loaded waveguide having an insulating transitionregion as described above, it is possible to simultaneously change thedistribution of carriers beneath the strip (i.e., in the slab) andbeneath also the “rulings” or strips 908 of the grating.

[0190] The various structures discussed above, offer a wide range ofadvantages and can be employed in a broad variety of applications. Forexample, tunable resonant cavities can be utilized for selectivelyfiltering one or more given optical frequencies. These resonant cavitiesmay include an active material in guiding to provide gain and to therebyform a laser. Thermal or other types of drift in the output frequency ofthe laser can be monitored and used as feedback to tune the resonantcavity in a fashion similar to that described above. These tunableresonators can be included together with a pair of waveguides tocontrollably couple light from one waveguide to another. Switching canbe performed in this manner. A light source can be modulated with such aconfiguration by directing light from the light source into one of thewaveguides. The output of either of the waveguides will correspond to amodulated optical signal depending on the modulation introduced by theresonant cavity. In this manner, information, voice or data, may beimparted on the optical signal. Switching can be implemented withdirectional coupler comprising two waveguides without any resonantfilter as described above. These filters, modulators, variable opticalcouplers, directional couplers and switches and various other devicesmay find use in optical communications and telecommunications but shouldnot limited to any particular application. Additionally, tunablewaveguide gratings can be created that allow the output angle,wavelength, and scattering strength, among other parameters, to bevaried and controlled.

[0191] As described above, silicon is substantially opticallytransmissive to certain wavelengths of interest such as 1.55 microns. Inaddition, processes for fabricating silicon structures are welldeveloped. For these reasons, waveguide structures comprisingpolysilicon and silicon are advantageous.

[0192] Although silicon is beneficial because it is substantiallytransparent at certain wavelengths, other materials and moreparticularly, other semiconductors may be employed as well. Furthermore,the structures described herein are not to be limited to any particularwavelength or wavelength range and may be designed, for example, formicrowave, infrared, visible, and ultraviolet wavelengths.

[0193] Those skilled in the art will appreciate that the methods anddesigns described above have additional applications and that therelevant applications are not limited to those specifically recitedabove. Also, the present invention may be embodied in other specificforms without departing from the essential characteristics as describedherein. The embodiments described above are to be considered in allrespects as illustrative only and not restrictive in any manner.

What is claimed is:
 1. A waveguide grating comprising a waveguide forpropagating light in a longitudinal direction, comprising a plurality ofelongate members oriented transverse to said longitudinal direction,said members disposed relative to said waveguide to form a grating forcoupling light out of the waveguide, said waveguide having a carrierdensity at each of said members, said members including respectiveelectrodes for applying an electric field to said waveguide, saidelectric field altering said carrier density in said waveguide such thatsaid coupling is altered.
 2. The waveguide grating of claim 1, whereinthe waveguide comprises semiconductor.
 3. The waveguide grating of claim2, wherein the waveguide comprises silicon.
 4. The waveguide grating ofclaim 2, further comprising an insulating layer disposed between saidelongated members and said waveguide.
 5. The waveguide grating of claim1, wherein said waveguide comprises a strip loaded waveguide.
 6. Thewaveguide grating of claim 1, wherein said strip loaded waveguidecomprises a slab, a strip formed thereon, and an intermediate layerbetween said strip and said slab.
 7. The waveguide grating of claim 6,wherein said slab comprises semiconductor and said intermediate layercomprises dielectric material.
 8. The waveguide grating of claim 7,further comprising at least one electrode juxtaposed with respect tosaid slab to induce an electric field through said dielectricintermediate layer and into said semiconductor slab to alter thedistribution of carriers in said semiconductor slab thereby altering theeffective index of said strip loaded waveguide.